Handle tile prioritization correctly when global visible rect is empty.

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 d9d3d0b1e18ab06b1eafcec9a9d45d6fd66dc6a8..af47e6f432a97416103d5db60d8cb9eeb0aa3aae 100644
--- a/android_webview/browser/browser_view_renderer.cc
+++ b/android_webview/browser/browser_view_renderer.cc
@@ -299,8 +299,14 @@ bool BrowserViewRenderer::OnDrawHardware(jobject java_canvas) {
   gfx::Rect viewport_rect_for_tile_priority;
   if (parent_draw_constraints_.is_layer)
     viewport_rect_for_tile_priority = parent_draw_constraints_.surface_rect;
-  else
+  else if (!last_on_draw_global_visible_rect_.IsEmpty())
     viewport_rect_for_tile_priority = last_on_draw_global_visible_rect_;
+  else {
+    // An empty viewport rect for tile priority will make cc fallback to use
+    // the WebView size as viewport rect for tile priority, which is not what
+    // we want. Instead, use the smallest non-empty rect for it.
+    viewport_rect_for_tile_priority = gfx::Rect(0, 0, 1, 1);
+  }
 
   scoped_ptr<cc::CompositorFrame> frame =
       compositor_->DemandDrawHw(surface_size,