webgl: Swap chain FinalizeFrame() fixes

1) Present swap chain before exporting back buffer in FinalizeFrame().
2) Check that drawing buffer is actually using a swap chain instead of
   just checking the runtime feature flag.

Bug: 939657
Change-Id: I235fe95c257c1e9971667e0e41c8d27bc27ad133
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1745404
Commit-Queue: Sunny Sachanandani <sunnyps@chromium.org>
Reviewed-by: Kai Ninomiya <kainino@chromium.org>
Reviewed-by: Kenneth Russell <kbr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#686601}

third_party/blink/renderer/core/html/canvas/canvas_rendering_context.cc

@@ -124,11 +124,13 @@ void CanvasRenderingContext::DidProcessTask(
     const base::PendingTask& /* pending_task */) {
   StopListeningForDidProcessTask();
 
+  // Call FinalizeFrame() on self first before informing host so that we can
+  // present swap chain before exporting it.
+  FinalizeFrame();
   // The end of a script task that drew content to the canvas is the point
   // at which the current frame may be considered complete.
   if (Host())
     Host()->FinalizeFrame();
-  FinalizeFrame();
 }
 
 CanvasRenderingContext::ContextType CanvasRenderingContext::ContextTypeFromId(

third_party/blink/renderer/core/html/canvas/html_canvas_element.cc

@@ -451,7 +451,7 @@ void HTMLCanvasElement::FinalizeFrame() {
     if (GetOrCreateCanvasResourceProvider(kPreferAcceleration)) {
       const bool webgl_overlay_enabled =
           RuntimeEnabledFeatures::WebGLImageChromiumEnabled() ||
-          RuntimeEnabledFeatures::WebGLSwapChainEnabled();
+          context_->UsingSwapChain();
       // TryEnableSingleBuffering() the first time we FinalizeFrame().
       if (!ResourceProvider()->IsSingleBuffered()) {
         ResourceProvider()->TryEnableSingleBuffering();

third_party/blink/renderer/platform/graphics/gpu/drawing_buffer.cc

@@ -1437,6 +1437,9 @@ void DrawingBuffer::PresentSwapChain() {
   DCHECK(UsingSwapChain());
   DCHECK_EQ(texture_target_, static_cast<unsigned>(GL_TEXTURE_2D));
 
+  if (!contents_changed_)
+    return;
+
   ScopedStateRestorer scoped_state_restorer(this);
   ResolveIfNeeded();
 
@@ -1472,6 +1475,7 @@ void DrawingBuffer::PresentSwapChain() {
                                 GL_FALSE, GL_FALSE);
   }
   ResetBuffersToAutoClear();
+  contents_changed_ = false;
 }
 
 scoped_refptr<DrawingBuffer::ColorBuffer> DrawingBuffer::CreateColorBuffer(