Discard compositor frames from unloaded web content

cc/ipc/struct_traits_unittest.cc

Index: cc/ipc/struct_traits_unittest.cc
diff --git a/cc/ipc/struct_traits_unittest.cc b/cc/ipc/struct_traits_unittest.cc
index 5b0686e8c92cdcb945b350dbb911fc9a13996fc8..c425247f604896994cb5bbeb553fd8b9ab9fde8b 100644
--- a/cc/ipc/struct_traits_unittest.cc
+++ b/cc/ipc/struct_traits_unittest.cc
@@ -240,6 +240,7 @@ TEST_F(StructTraitsTest, CompositorFrame) {
   const gfx::Vector2dF root_scroll_offset(1234.5f, 6789.1f);
   const float page_scale_factor = 1337.5f;
   const gfx::SizeF scrollable_viewport_size(1337.7f, 1234.5f);
+  const uint32_t content_source_id = 3;
 
   CompositorFrame input;
   input.metadata.device_scale_factor = device_scale_factor;
@@ -248,6 +249,7 @@ TEST_F(StructTraitsTest, CompositorFrame) {
   input.metadata.scrollable_viewport_size = scrollable_viewport_size;
   input.render_pass_list.push_back(std::move(render_pass));
   input.resource_list.push_back(resource);
+  input.metadata.content_source_id = content_source_id;
 
   mojom::TraitsTestServicePtr proxy = GetTraitsTestProxy();
   CompositorFrame output;
@@ -257,6 +259,7 @@ TEST_F(StructTraitsTest, CompositorFrame) {
   EXPECT_EQ(root_scroll_offset, output.metadata.root_scroll_offset);
   EXPECT_EQ(page_scale_factor, output.metadata.page_scale_factor);
   EXPECT_EQ(scrollable_viewport_size, output.metadata.scrollable_viewport_size);
+  EXPECT_EQ(content_source_id, output.metadata.content_source_id);
 
   ASSERT_EQ(1u, output.resource_list.size());
   TransferableResource out_resource = output.resource_list[0];