Issue #1925 - Rename Corner to LogicalCorner and move it to nsStyleCoord.h.

To avoid confusion with mozilla::css::corner, rename ::Corner to
mozilla::LogicalCorner, and move it to nsStyleCoord.h from the illogical
nsCellMap.h location.
Also, append the LogicalCorner prefix to all the enum values, to match
the surrounding coding style.

layout/style/nsStyleCoord.h

@@ -15,7 +15,7 @@ namespace mozilla {
 
 class WritingMode;
 
-// Logical axis, edge and side constants for use in various places.
+// Logical axis, edge, side and corner constants for use in various places.
 enum LogicalAxis {
   eLogicalAxisBlock  = 0x0,
   eLogicalAxisInline = 0x1
@@ -31,6 +31,14 @@ enum LogicalSide {
   eLogicalSideIEnd   = (eLogicalAxisInline << 1) | eLogicalEdgeEnd     // 0x3
 };
 
+enum LogicalCorner
+{
+  eLogicalCornerBStartIStart = 0,
+  eLogicalCornerBStartIEnd   = 1,
+  eLogicalCornerBEndIEnd     = 2,
+  eLogicalCornerBEndIStart   = 3
+};
+
 } // namespace mozilla
 
 enum nsStyleUnit : uint8_t {