Avoid to resolve CMAA for read and draw without content changed or with content committed.

CMAA will be triggered by every readPixels call, even content does not be  changed or committed.

BUG=535198

TEST=conformance/readinging/fbo-remains-unchanged-after-read-pixels.html

Review-Url: https://codereview.chromium.org/2597933002
Cr-Commit-Position: refs/heads/master@{#467915}

third_party/WebKit/Source/platform/graphics/gpu/DrawingBuffer.cpp

@@ -992,7 +992,7 @@ void DrawingBuffer::ResolveAndBindForReadAndDraw() {
 void DrawingBuffer::ResolveMultisampleFramebufferInternal() {
   DCHECK(state_restorer_);
   state_restorer_->SetFramebufferBindingDirty();
-  if (WantExplicitResolve() && !contents_change_resolved_) {
+  if (WantExplicitResolve()) {
     state_restorer_->SetClearStateDirty();
     gl_->BindFramebuffer(GL_READ_FRAMEBUFFER_ANGLE, multisample_fbo_);
     gl_->BindFramebuffer(GL_DRAW_FRAMEBUFFER_ANGLE, fbo_);
@@ -1025,7 +1025,7 @@ void DrawingBuffer::ResolveMultisampleFramebufferInternal() {
 }
 
 void DrawingBuffer::ResolveIfNeeded() {
-  if (anti_aliasing_mode_ != kNone)
+  if (anti_aliasing_mode_ != kNone && !contents_change_resolved_)
     ResolveMultisampleFramebufferInternal();
   contents_change_resolved_ = true;
 }