Update comments regarding image caching

While attempting to remove the layoutObject parameter the following
observations were made:

 * The decoding forced by BitmapImage::imageForCurrentFrame will be
   deferred so the comment in the header may be removed.

 * BitmapImage::currentFrameKnownToBeOpaque() conservatively returns
   false for uncached images, not true.

 * Since decoding is deferred, there is no guarantee that the
   opaqueness metadata is available after the imageForCurrentFrame
   call has returned. It may increase the chance though, depending on
   image decoder for the particular image.

NOTRY=true
BUG=559131

Review URL: https://codereview.chromium.org/1454373005

Cr-Commit-Position: refs/heads/master@{#361094}

third_party/WebKit/Source/core/fetch/ImageResource.cpp

@@ -451,8 +451,9 @@ bool ImageResource::currentFrameKnownToBeOpaque(const LayoutObject* layoutObject
     blink::Image* image = this->image();
     if (image->isBitmapImage()) {
         TRACE_EVENT1(TRACE_DISABLED_BY_DEFAULT("devtools.timeline"), "PaintImage", "data", InspectorPaintImageEvent::data(layoutObject, *this));
-        // BitmapImage::currentFrameKnownToBeOpaque() conservatively returns true for uncached
-        // frames. To get an accurate answer, we pre-cache the current frame metadata.
+        // BitmapImage::currentFrameKnownToBeOpaque() conservatively returns false for uncached
+        // frames. To increase the change of an accurate answer, we pre-cache the current frame
+        // metadata.
         image->imageForCurrentFrame();
     }
     return image->currentFrameKnownToBeOpaque();

third_party/WebKit/Source/core/fetch/ImageResource.h

@@ -60,8 +60,6 @@ public:
     blink::Image* image(); // Returns the nullImage() if the image is not available yet.
     bool hasImage() const { return m_image.get(); }
     // Side effect: ensures decoded image is in cache, therefore should only be called when about to draw the image.
-    // FIXME: Decoding image on the main thread is expensive, so rather than forcing decode, consider returning false
-    // when image is not decoded yet, as we do in case of deferred decoding.
     bool currentFrameKnownToBeOpaque(const LayoutObject*);
 
     static std::pair<blink::Image*, float> brokenImage(float deviceScaleFactor); // Returns an image and the image's resolution scale factor.