cc: Invalidate for needs_prepare_tiles

Problem is when CanDraw is false but needs_prepare_tiles is true
Scheduler will keep requesting begin frames but do nothing in a
begin frame since invalidate is blocked on CanDraw.

We already have an efficient shortcut that does not involve actually
producing a frame if invalidate is only requested for
needs_prepare_tiles. So the simple fix is simply allow invalidates for
this case.

Bug: 900557
Change-Id: Ib3c3e213e2e2461d7aa1b8dd8d2e91ab64042feb
Reviewed-on: https://chromium-review.googlesource.com/c/1334550
Commit-Queue: Bo <boliu@chromium.org>
Reviewed-by: Sunny Sachanandani <sunnyps@chromium.org>
Cr-Commit-Position: refs/heads/master@{#607876}

cc/scheduler/scheduler_state_machine.cc

@@ -565,14 +565,12 @@ bool SchedulerStateMachine::ShouldInvalidateLayerTreeFrameSink() const {
   if (begin_impl_frame_state_ != BeginImplFrameState::INSIDE_BEGIN_FRAME)
     return false;
 
-  // Don't invalidate if we cannnot draw.
-  if (PendingDrawsShouldBeAborted())
-    return false;
-
+  // Don't invalidate for draw if we cannnot draw.
   // TODO(sunnyps): needs_prepare_tiles_ is needed here because PrepareTiles is
   // called only inside the deadline / draw phase. We could remove this if we
   // allowed PrepareTiles to happen in OnBeginImplFrame.
-  return needs_redraw_ || needs_prepare_tiles_;
+  return (needs_redraw_ && !PendingDrawsShouldBeAborted()) ||
+         needs_prepare_tiles_;
 }
 
 SchedulerStateMachine::Action SchedulerStateMachine::NextAction() const {

cc/scheduler/scheduler_unittest.cc

@@ -3091,6 +3091,25 @@ TEST_F(SchedulerTest, InvalidateLayerTreeFrameSinkWhenCannotDraw) {
   EXPECT_FALSE(scheduler_->RedrawPending());
 }
 
+TEST_F(SchedulerTest, NeedsPrepareTilesInvalidates) {
+  // This is to test that SetNeedsPrepareTiles causes invalidates even if
+  // CanDraw is false.
+  scheduler_settings_.using_synchronous_renderer_compositor = true;
+  SetUpScheduler(EXTERNAL_BFS);
+
+  scheduler_->SetCanDraw(false);
+
+  scheduler_->SetNeedsPrepareTiles();
+  EXPECT_ACTIONS("AddObserver(this)");
+  client_->Reset();
+
+  // Do not invalidate in next BeginFrame.
+  EXPECT_SCOPED(AdvanceFrame());
+  EXPECT_ACTIONS("WillBeginImplFrame",
+                 "ScheduledActionInvalidateLayerTreeFrameSink");
+  client_->Reset();
+}
+
 TEST_F(SchedulerTest, SetNeedsOneBeginImplFrame) {
   SetUpScheduler(EXTERNAL_BFS);