Revert of Remove fallback tick software draw.

android_webview/browser/browser_view_renderer.cc

Index: android_webview/browser/browser_view_renderer.cc
diff --git a/android_webview/browser/browser_view_renderer.cc b/android_webview/browser/browser_view_renderer.cc
index c61f2e16aded49309d188b7e9fc803a3c02d9b8a..fa9ea172dc883e145060c237771bcb75d6cb64ea 100644
--- a/android_webview/browser/browser_view_renderer.cc
+++ b/android_webview/browser/browser_view_renderer.cc
@@ -30,6 +30,8 @@ namespace {
 
 const double kEpsilon = 1e-8;
 
+const int64 kFallbackTickTimeoutInMilliseconds = 100;
+
 // Used to calculate memory allocation. Determined experimentally.
 const size_t kMemoryMultiplier = 20;
 const size_t kBytesPerPixel = 4;
@@ -101,7 +103,8 @@ BrowserViewRenderer::BrowserViewRenderer(
       max_page_scale_factor_(0.f),
       on_new_picture_enable_(false),
       clear_view_(false),
-      offscreen_pre_raster_(false) {}
+      offscreen_pre_raster_(false),
+      fallback_tick_pending_(false) {}
 
 BrowserViewRenderer::~BrowserViewRenderer() {
 }
@@ -222,6 +225,12 @@ bool BrowserViewRenderer::OnDrawHardware() {
   shared_renderer_state_.SetScrollOffsetOnUI(last_on_draw_scroll_offset_);
   hardware_enabled_ = true;
 
+  return CompositeHw();
+}
+
+bool BrowserViewRenderer::CompositeHw() {
+  CancelFallbackTick();
+
   ReturnResourceFromParent();
   UpdateMemoryPolicy();
 
@@ -261,16 +270,14 @@ bool BrowserViewRenderer::OnDrawHardware() {
                      transform_for_tile_priority, offscreen_pre_raster_,
                      parent_draw_constraints.is_layer));
 
-  // If we haven't received a kModeSync functor call since the last
-  // CompositeHw then we need to discard the resources that are being
-  // held onto by the previously prepared frame.
+  // Uncommitted frame can happen with consecutive fallback ticks.
   ReturnUnusedResource(shared_renderer_state_.PassUncommittedFrameOnUI());
   shared_renderer_state_.SetCompositorFrameOnUI(child_frame.Pass());
   return true;
 }
 
 void BrowserViewRenderer::UpdateParentDrawConstraints() {
-  PostInvalidate();
+  PostInvalidateWithFallback();
   ParentCompositorDrawConstraints parent_draw_constraints =
       shared_renderer_state_.GetParentDrawConstraintsOnUI();
   client_->ParentDrawConstraintsUpdated(parent_draw_constraints);
@@ -344,7 +351,7 @@ void BrowserViewRenderer::ClearView() {
 
   clear_view_ = true;
   // Always invalidate ignoring the compositor to actually clear the webview.
-  PostInvalidate();
+  PostInvalidateWithFallback();
 }
 
 void BrowserViewRenderer::SetOffscreenPreRaster(bool enable) {
@@ -642,11 +649,80 @@ void BrowserViewRenderer::DidOverscroll(
 void BrowserViewRenderer::PostInvalidate() {
   TRACE_EVENT_INSTANT0("android_webview", "BrowserViewRenderer::PostInvalidate",
                        TRACE_EVENT_SCOPE_THREAD);
+  PostInvalidateWithFallback();
+}
+
+void BrowserViewRenderer::PostInvalidateWithFallback() {
+  // Always call view invalidate. We rely the Android framework to ignore the
+  // invalidate when it's not needed such as when view is not visible.
   client_->PostInvalidate();
+
+  // Stop fallback ticks when one of these is true.
+  // 1) Webview is paused. Also need to check we are not in clear view since
+  // paused, offscreen still expect clear view to recover.
+  // 2) If we are attached to window and the window is not visible (eg when
+  // app is in the background). We are sure in this case the webview is used
+  // "on-screen" but that updates are not needed when in the background.
+  bool throttle_fallback_tick =
+      (is_paused_ && !clear_view_) || (attached_to_window_ && !window_visible_);
+
+  if (throttle_fallback_tick || fallback_tick_pending_)
+    return;
+
+  DCHECK(post_fallback_tick_.IsCancelled());
+  DCHECK(fallback_tick_fired_.IsCancelled());
+
+  post_fallback_tick_.Reset(base::Bind(&BrowserViewRenderer::PostFallbackTick,
+                                       base::Unretained(this)));
+  ui_task_runner_->PostTask(FROM_HERE, post_fallback_tick_.callback());
+  fallback_tick_pending_ = true;
+}
+
+void BrowserViewRenderer::CancelFallbackTick() {
+  post_fallback_tick_.Cancel();
+  fallback_tick_fired_.Cancel();
+  fallback_tick_pending_ = false;
+}
+
+void BrowserViewRenderer::PostFallbackTick() {
+  DCHECK(fallback_tick_fired_.IsCancelled());
+  TRACE_EVENT0("android_webview", "BrowserViewRenderer::PostFallbackTick");
+  post_fallback_tick_.Cancel();
+  fallback_tick_fired_.Reset(base::Bind(&BrowserViewRenderer::FallbackTickFired,
+                                        base::Unretained(this)));
+  ui_task_runner_->PostDelayedTask(
+      FROM_HERE, fallback_tick_fired_.callback(),
+      base::TimeDelta::FromMilliseconds(kFallbackTickTimeoutInMilliseconds));
+}
+
+void BrowserViewRenderer::FallbackTickFired() {
+  TRACE_EVENT0("android_webview", "BrowserViewRenderer::FallbackTickFired");
+  // This should only be called if OnDraw or DrawGL did not come in time, which
+  // means fallback_tick_pending_ must still be true.
+  DCHECK(fallback_tick_pending_);
+  fallback_tick_fired_.Cancel();
+  fallback_tick_pending_ = false;
+  if (compositor_) {
+    if (hardware_enabled_ && !size_.IsEmpty()) {
+      CompositeHw();
+    } else {
+      ForceFakeCompositeSW();
+    }
+  }
+}
+
+void BrowserViewRenderer::ForceFakeCompositeSW() {
+  DCHECK(compositor_);
+  SkBitmap bitmap;
+  bitmap.allocN32Pixels(1, 1);
+  bitmap.eraseColor(0);
+  SkCanvas canvas(bitmap);
+  CompositeSW(&canvas);
 }
 
 bool BrowserViewRenderer::CompositeSW(SkCanvas* canvas) {
   DCHECK(compositor_);
+  CancelFallbackTick();
   ReturnResourceFromParent();
   return compositor_->DemandDrawSw(canvas);
 }
@@ -668,6 +744,8 @@ std::string BrowserViewRenderer::ToString() const {
   base::StringAppendF(&str, "window_visible: %d ", window_visible_);
   base::StringAppendF(&str, "dip_scale: %f ", dip_scale_);
   base::StringAppendF(&str, "page_scale_factor: %f ", page_scale_factor_);
+  base::StringAppendF(&str, "fallback_tick_pending: %d ",
+                      fallback_tick_pending_);
   base::StringAppendF(&str, "view size: %s ", size_.ToString().c_str());
   base::StringAppendF(&str, "attached_to_window: %d ", attached_to_window_);
   base::StringAppendF(&str,