Cherry-pick: Do not clear root frame buffer for android webview

Cherry-pick of unlanded https://codereview.chromium.org/210443004/

Original description:

Android webview does not own the surface that compositor is drawing
into, and so should in general not it before draw.

BUG: 13134274
Change-Id: I90e8f39bd455f2494dbdcf48a40f15f837cf76fb
diff --git a/content/renderer/gpu/render_widget_compositor.cc b/content/renderer/gpu/render_widget_compositor.cc
index 90ae2fe..b3551d5 100644
--- a/content/renderer/gpu/render_widget_compositor.cc
+++ b/content/renderer/gpu/render_widget_compositor.cc
@@ -271,6 +271,9 @@
       base::android::SysUtils::IsLowEndDevice() &&
       !widget->UsingSynchronousRendererCompositor() &&
       !cmd->HasSwitch(cc::switches::kDisable4444Textures);
+  // Webview does not own the surface so should not clear it.
+  settings.should_clear_root_render_pass =
+      !widget->UsingSynchronousRendererCompositor();
 #elif !defined(OS_MACOSX)
   if (cmd->HasSwitch(switches::kEnableOverlayScrollbars)) {
     settings.scrollbar_animator = cc::LayerTreeSettings::Thinning;