Make opportunistic encryption configurable.

This adds a pref "network.http.opportunistic-encryption" that controls whether
we send an "Upgrade-Insecure-Requests : 1" header on document navigation or not.
This patch modifies the platform network parts. Default for the platform is "true".
Part 1 for #863

modules/libpref/init/all.js

@@ -1374,6 +1374,9 @@ pref("network.tickle-wifi.delay", 16);
 // Turn off interprocess security checks. Needed to run xpcshell tests.
 pref("network.disable.ipc.security", false);
 
+// Send upgrade-insecure-requests HTTP header
+pref("network.http.opportunistic-encryption", true);
+
 // Default action for unlisted external protocol handlers
 pref("network.protocol-handler.external-default", true);      // OK to load
 pref("network.protocol-handler.warn-external-default", true); // warn before load

netwerk/protocol/http/nsHttpChannel.cpp

@@ -313,11 +313,15 @@ nsHttpChannel::nsHttpChannel()
     , mPushedStream(nullptr)
     , mLocalBlocklist(false)
     , mWarningReporter(nullptr)
+    , mSendUpgradeRequest(false)
     , mDidReval(false)
 {
     LOG(("Creating nsHttpChannel [this=%p]\n", this));
     mChannelCreationTime = PR_Now();
     mChannelCreationTimestamp = TimeStamp::Now();
+    
+    mSendUpgradeRequest = 
+      Preferences::GetBool("network.http.opportunistic-encryption", false);
 }
 
 nsHttpChannel::~nsHttpChannel()
@@ -377,8 +381,9 @@ nsHttpChannel::Connect()
                                mLoadInfo->GetExternalContentPolicyType() :
                                nsIContentPolicy::TYPE_OTHER;
 
-    if (type == nsIContentPolicy::TYPE_DOCUMENT ||
-        type == nsIContentPolicy::TYPE_SUBDOCUMENT) {
+    if (mSendUpgradeRequest &&
+        (type == nsIContentPolicy::TYPE_DOCUMENT ||
+         type == nsIContentPolicy::TYPE_SUBDOCUMENT)) {
         rv = SetRequestHeader(NS_LITERAL_CSTRING("Upgrade-Insecure-Requests"),
                               NS_LITERAL_CSTRING("1"), false);
         NS_ENSURE_SUCCESS(rv, rv);

netwerk/protocol/http/nsHttpChannel.h

@@ -597,6 +597,10 @@ private:
     HttpChannelSecurityWarningReporter* mWarningReporter;
 
     RefPtr<ADivertableParentChannel> mParentChannel;
+    
+    // Whether we send opportunistic encryption requests.
+    bool mSendUpgradeRequest;
+
 protected:
     virtual void DoNotifyListenerCleanup() override;