Only do PDF printing on cleared Canvases

Otherwise we may lose back information

Bug: 1011309
Change-Id: Idabededd27383bca54db237a23fbd245b8c83eae
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1845231
Commit-Queue: Fernando Serboncini <fserb@chromium.org>
Reviewed-by: Juanmi Huertas <juanmihd@chromium.org>
Cr-Commit-Position: refs/heads/master@{#703392}

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

@@ -583,6 +583,8 @@ void HTMLCanvasElement::Reset() {
   if (had_resource_provider && old_size == new_size && Is2d()) {
     if (!canvas_is_clear_) {
       canvas_is_clear_ = true;
+      if (canvas2d_bridge_)
+        canvas2d_bridge_->ClearFrame();
       context_->ClearRect(0, 0, width(), height());
     }
     return;

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

@@ -575,9 +575,10 @@ void Canvas2DLayerBridge::FlushRecording() {
     cc::PaintCanvas* canvas = ResourceProvider()->Canvas();
     last_recording_ = recorder_->finishRecordingAsPicture();
     canvas->drawPicture(last_recording_);
-    if (!resource_host_ || !resource_host_->IsPrinting() ||
+    if (!clear_frame_ || !resource_host_ || !resource_host_->IsPrinting() ||
         !is_deferral_enabled_) {
       last_recording_ = nullptr;
+      clear_frame_ = false;
     }
     ResourceProvider()->FlushSkia();
   }

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

@@ -172,6 +172,10 @@ class PLATFORM_EXPORT Canvas2DLayerBridge : public cc::TextureLayerClient {
 
   sk_sp<cc::PaintRecord> getLastRecord() { return last_recording_; }
 
+  // This is called when the Canvas element has cleared the frame, so the 2D
+  // bridge knows that there's no previous content on the resource.
+  void ClearFrame() { clear_frame_ = true; }
+
  private:
   friend class Canvas2DLayerBridgeTest;
   friend class CanvasRenderingContext2DTest;
@@ -199,6 +203,7 @@ class PLATFORM_EXPORT Canvas2DLayerBridge : public cc::TextureLayerClient {
   bool hibernation_scheduled_ = false;
   bool dont_use_idle_scheduling_for_testing_ = false;
   bool context_lost_ = false;
+  bool clear_frame_ = true;
 
   const AccelerationMode acceleration_mode_;
   const CanvasColorParams color_params_;