Update popup background-color to avoid white flash on popup open.

This CL updates the default background-color of the popup to
transparent instead of white.

Doing this we avoid having a white flash on popup open when
dark color-scheme is used, as the default background color of the
popup is white.

For more details: http://crrev.com/c/2517730.

Bug: 1114272
Change-Id: Ia9676bba7c343bd0b18f9be4ea38d94dcacc3899
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2525601Reviewed-by: Rune Lillesveen <futhark@chromium.org>
Reviewed-by: Avi Drissman <avi@chromium.org>
Commit-Queue: Ionel Popescu <iopopesc@microsoft.com>
Cr-Commit-Position: refs/heads/master@{#825509}

content/browser/renderer_host/render_widget_host_view_aura.cc

@@ -375,6 +375,9 @@ void RenderWidgetHostViewAura::InitAsPopup(
   }
   popup_parent_host_view_->SetPopupChild(this);
   CreateAuraWindow(aura::client::WINDOW_TYPE_MENU);
+  // Use transparent background color for the popup in order to avoid flashing
+  // the white background on popup open when dark color-scheme is used.
+  SetContentBackgroundColor(SK_ColorTRANSPARENT);
 
   // Setting the transient child allows for the popup to get mouse events when
   // in a system modal dialog. Do this before calling ParentWindowWithContext

content/browser/renderer_host/render_widget_host_view_mac.mm

@@ -356,6 +356,10 @@ void RenderWidgetHostViewMac::InitAsPopup(
   }
   popup_parent_host_view_->popup_child_host_view_ = this;
 
+  // Use transparent background color for the popup in order to avoid flashing
+  // the white background on popup open when dark color-scheme is used.
+  SetContentBackgroundColor(SK_ColorTRANSPARENT);
+
   // This path is used by the time/date picker.
   // When FormControlsRefresh is enabled the popup window should use
   // the native shadow.