pt 1 in reviving the android build (copied from pm 28a1, wish me luck)

mobile/android/chrome/content/MemoryObserver.js

@@ -0,0 +1,88 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this file,
+ * You can obtain one at http://mozilla.org/MPL/2.0/. */
+"use strict";
+
+var MemoryObserver = {
+  observe: function mo_observe(aSubject, aTopic, aData) {
+    if (aTopic == "memory-pressure") {
+      if (aData != "heap-minimize") {
+        this.handleLowMemory();
+      }
+      // The JS engine would normally GC on this notification, but since we
+      // disabled that in favor of this method (bug 669346), we should gc here.
+      // See bug 784040 for when this code was ported from XUL to native Fennec.
+      this.gc();
+    } else if (aTopic == "Memory:Dump") {
+      this.dumpMemoryStats(aData);
+    }
+  },
+
+  handleLowMemory: function() {
+    // do things to reduce memory usage here
+    if (!Services.prefs.getBoolPref("browser.tabs.disableBackgroundZombification")) {
+      let tabs = BrowserApp.tabs;
+      let selected = BrowserApp.selectedTab;
+      for (let i = 0; i < tabs.length; i++) {
+        if (tabs[i] != selected && !tabs[i].playingAudio) {
+          this.zombify(tabs[i]);
+        }
+      }
+    }
+
+    // Change some preferences temporarily for only this session
+    let defaults = Services.prefs.getDefaultBranch(null);
+
+    // Reduce the amount of decoded image data we keep around
+    defaults.setIntPref("image.mem.max_decoded_image_kb", 0);
+
+    // Stop using the bfcache
+    if (!Services.prefs.getBoolPref("browser.sessionhistory.bfcacheIgnoreMemoryPressure")) {
+      defaults.setIntPref("browser.sessionhistory.max_total_viewers", 0);
+    }
+  },
+
+  zombify: function(tab) {
+    let browser = tab.browser;
+    let data = browser.__SS_data;
+    let extra = browser.__SS_extdata;
+
+    // Notify any interested parties (e.g. the session store)
+    // that the original tab object is going to be destroyed
+    let evt = document.createEvent("UIEvents");
+    evt.initUIEvent("TabPreZombify", true, false, window, null);
+    browser.dispatchEvent(evt);
+
+    // We need this data to correctly create and position the new browser
+    // If this browser is already a zombie, fallback to the session data
+    let currentURL = browser.__SS_restore ? data.entries[0].url : browser.currentURI.spec;
+    let sibling = browser.nextSibling;
+    let isPrivate = PrivateBrowsingUtils.isBrowserPrivate(browser);
+
+    tab.destroy();
+    tab.create(currentURL, { sibling: sibling, zombifying: true, delayLoad: true, isPrivate: isPrivate });
+
+    // Reattach session store data and flag this browser so it is restored on select
+    browser = tab.browser;
+    browser.__SS_data = data;
+    browser.__SS_extdata = extra;
+    browser.__SS_restore = true;
+    browser.setAttribute("pending", "true");
+
+    // Notify the session store to reattach its listeners to the new tab object
+    evt = document.createEvent("UIEvents");
+    evt.initUIEvent("TabPostZombify", true, false, window, null);
+    browser.dispatchEvent(evt);
+  },
+
+  gc: function() {
+    window.QueryInterface(Ci.nsIInterfaceRequestor).getInterface(Ci.nsIDOMWindowUtils).garbageCollect();
+    Cu.forceGC();
+  },
+
+  dumpMemoryStats: function(aLabel) {
+    let memDumper = Cc["@mozilla.org/memory-info-dumper;1"].getService(Ci.nsIMemoryInfoDumper);
+    memDumper.dumpMemoryInfoToTempDir(aLabel, /* anonymize = */ false,
+                                      /* minimize = */ false);
+  },
+};