Bug 1341703 - Part 3. Handle nsStyleBorder::CalcDifference off main thread. r=heycam

Image loading status can be accessed in main thread only, so we can not use
nsStyleImage::IsLoaded in nsStyleBorder::CalcDifference, which might be executed
on a background thread.

The drawback of this change is we may paint a border-image, once, even before it
was actually loaded, which is a waste, because we have nothing to draw yet.

PS:
We did not check nsStyleImage::IsLoaded in both background-image and mask-image's
CalcDifference.

MozReview-Commit-ID: 5knH6dZziL8

--HG--
extra : rebase_source : 8e854960bfec130fe06e57b5e898651814e7f36e

layout/style/nsStyleStruct.cpp

@@ -479,7 +479,11 @@ nsStyleBorder::CalcDifference(const nsStyleBorder& aNewData) const
     return nsChangeHint_RepaintFrame;
   }
 
-  if (IsBorderImageLoaded() || aNewData.IsBorderImageLoaded()) {
+  // Loading status of the border image can be accessed in main thread only
+  // while CalcDifference might be executed on a background thread. As a
+  // result, we have to check mBorderImage* fields even before border image was
+  // actually loaded.
+  if (!mBorderImageSource.IsEmpty() || !aNewData.mBorderImageSource.IsEmpty()) {
     if (mBorderImageSource  != aNewData.mBorderImageSource  ||
         mBorderImageRepeatH != aNewData.mBorderImageRepeatH ||
         mBorderImageRepeatV != aNewData.mBorderImageRepeatV ||
@@ -508,9 +512,7 @@ nsStyleBorder::CalcDifference(const nsStyleBorder& aNewData) const
     return nsChangeHint_NeutralChange;
   }
 
-  // mBorderImage* fields are checked only when border image was
-  // actualy loaded. But we need to return neutral change even when
-  // they are not actually used.
+  // mBorderImage* fields are checked only when border-image is not 'none'.
   if (mBorderImageSource  != aNewData.mBorderImageSource  ||
       mBorderImageRepeatH != aNewData.mBorderImageRepeatH ||
       mBorderImageRepeatV != aNewData.mBorderImageRepeatV ||