Make the custom launcher page's WebContents transparent.

This CL makes the custom launcher page's background transparent. This
means that custom launcher pages won't need to match their background
color to the app list.

This CL also fixes an issue with a white background rendering when the
view is clipping to the bounds.

BUG=425444

Review URL: https://codereview.chromium.org/720803005

Cr-Commit-Position: refs/heads/master@{#304747}

chrome/browser/ui/app_list/app_list_view_delegate.cc

@@ -40,6 +40,8 @@
 #include "content/public/browser/browser_thread.h"
 #include "content/public/browser/notification_service.h"
 #include "content/public/browser/page_navigator.h"
+#include "content/public/browser/render_view_host.h"
+#include "content/public/browser/render_widget_host_view.h"
 #include "content/public/browser/user_metrics.h"
 #include "content/public/browser/web_contents.h"
 #include "extensions/browser/extension_registry.h"
@@ -635,6 +637,10 @@ std::vector<views::View*> AppListViewDelegate::CreateCustomPageWebViews(
     // The web contents should belong to the current profile.
     DCHECK_EQ(profile_, web_contents->GetBrowserContext());
 
+    // Make the webview transparent.
+    web_contents->GetRenderViewHost()->GetView()->SetBackgroundColor(
+        SK_ColorTRANSPARENT);
+
     views::WebView* web_view =
         new views::WebView(web_contents->GetBrowserContext());
     web_view->SetPreferredSize(size);

ui/views/controls/native/native_view_host.cc

@@ -118,7 +118,7 @@ void NativeViewHost::OnPaint(gfx::Canvas* canvas) {
   //
   // It would be nice if this used some approximation of the page's
   // current background color.
-  if (native_wrapper_->HasInstalledClip())
+  if (fast_resize_ && native_wrapper_->HasInstalledClip())
     canvas->FillRect(GetLocalBounds(), SK_ColorWHITE);
 }