Splitting DidSwap in cc::SwapPromise into WillSwap and DidSwap

blimp/client/core/compositor/blimp_compositor.cc

Index: blimp/client/core/compositor/blimp_compositor.cc
diff --git a/blimp/client/core/compositor/blimp_compositor.cc b/blimp/client/core/compositor/blimp_compositor.cc
index edafad482814c074fa7fddfe9e1c665d7a0e44df..ec80025d6b74ec8ff79793958e77e36045c863c1 100644
--- a/blimp/client/core/compositor/blimp_compositor.cc
+++ b/blimp/client/core/compositor/blimp_compositor.cc
@@ -73,10 +73,10 @@ class BlimpCompositor::FrameTrackingSwapPromise : public cc::SwapPromise {
 
   // cc::SwapPromise implementation.
   void DidActivate() override {}
-  void DidSwap(cc::CompositorFrameMetadata* metadata) override {
-    // DidSwap is called right before the CompositorFrame is submitted to the
-    // CompositorFrameSink, so we make sure to delay the copy request till that
-    // frame is submitted.
+  void WillSwap(cc::CompositorFrameMetadata* metadata) override {}
+  void DidSwap() override {
+    // DidSwap could be called on compositor thread and we need this to run on
+    // the main thread.
     main_task_runner_->PostTask(
         FROM_HERE,
         base::Bind(&BlimpCompositor::MakeCopyRequestOnNextSwap,