aw: Ensure fallback tick unsets |block_invalidates_|

Currently, if |compositor_needs_continuous_invalidate_| is unset before
PostFallbackTick or FallbackTickFired, then the fallback ticks are
cancelled, but we never unset |block_invalidates_|, which will block
invalidates indefinitely.

Fix is simply always unset |block_invalidates_| even if we skip
the fallback tick.

BUG=408779

Review URL: https://codereview.chromium.org/512403002

Cr-Commit-Position: refs/heads/master@{#292511}

android_webview/browser/browser_view_renderer.cc

@@ -717,6 +717,9 @@ void BrowserViewRenderer::PostFallbackTick() {
         FROM_HERE,
         fallback_tick_fired_.callback(),
         base::TimeDelta::FromMilliseconds(kFallbackTickTimeoutInMilliseconds));
+  } else {
+    // Pretend we just composited to unblock further invalidates.
+    DidComposite();
   }
 }
 
@@ -729,8 +732,12 @@ void BrowserViewRenderer::FallbackTickFired() {
   // This should only be called if OnDraw or DrawGL did not come in time, which
   // means block_invalidates_ must still be true.
   DCHECK(block_invalidates_);
-  if (compositor_needs_continuous_invalidate_ && compositor_)
+  if (compositor_needs_continuous_invalidate_ && compositor_) {
     ForceFakeCompositeSW();
+  } else {
+    // Pretend we just composited to unblock further invalidates.
+    DidComposite();
+  }
 }
 
 void BrowserViewRenderer::ForceFakeCompositeSW() {