Canvas: always updated canvas content retainence |mode_|

In my previous cl, "Discard canvas content from frame if canvas is fully
repaint", I set the |mode_| to discard canvas content from previous
frame if there is some paint command in the current frame. These two are
not related. So I removed the dependence in this cl.

Bug: 1054666

Change-Id: Icc47a03aced6b0033cfe1b9bc5267aa514b8c456
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2088122
Commit-Queue: Yi Xu <yiyix@chromium.org>
Reviewed-by: Khushal <khushalsagar@chromium.org>
Cr-Commit-Position: refs/heads/master@{#747110}

third_party/blink/renderer/platform/graphics/canvas_2d_layer_bridge.cc

@@ -482,10 +482,8 @@ bool Canvas2DLayerBridge::WritePixels(const SkImageInfo& orig_info,
 }
 
 void Canvas2DLayerBridge::SkipQueuedDrawCommands() {
-  if (have_recorded_draw_commands_) {
   ResourceProvider()->SkipQueuedDrawCommands();
   have_recorded_draw_commands_ = false;
-  }
 
   if (rate_limiter_)
     rate_limiter_->Reset();

third_party/blink/renderer/platform/graphics/canvas_resource_provider.cc

@@ -1278,9 +1278,12 @@ scoped_refptr<CanvasResource> CanvasResourceProvider::GetImportedResource()
 }
 
 void CanvasResourceProvider::SkipQueuedDrawCommands() {
-  if (!recorder_)
-    return;
+  // Note that this function only gets called when canvas needs a full repaint,
+  // so always update the |mode_| to discard the old copy of canvas content.
   mode_ = SkSurface::kDiscard_ContentChangeMode;
+
+  if (!recorder_ || !recorder_->ListHasDrawOps())
+    return;
   recorder_->finishRecordingAsPicture();
   cc::PaintCanvas* canvas =
       recorder_->beginRecording(Size().Width(), Size().Height());