[Mac] Make app window backgrounds white.

chrome/browser/ui/cocoa/apps/native_app_window_cocoa.mm

Index: chrome/browser/ui/cocoa/apps/native_app_window_cocoa.mm
diff --git a/chrome/browser/ui/cocoa/apps/native_app_window_cocoa.mm b/chrome/browser/ui/cocoa/apps/native_app_window_cocoa.mm
index a4ad4eabc04d88a0f14caf214ffa876118597cd7..e7e25049551dfa144a5896025a19319bcbe2c4ad 100644
--- a/chrome/browser/ui/cocoa/apps/native_app_window_cocoa.mm
+++ b/chrome/browser/ui/cocoa/apps/native_app_window_cocoa.mm
@@ -217,6 +217,18 @@ std::vector<gfx::Rect> CalculateNonDraggableRegions(
   return YES;
 }
 
+- (void)drawCustomFrameRect:(NSRect)rect forView:(NSView*)view {
+  [super drawCustomFrameRect:rect
+                     forView:view];
+
+  // Make the background color of the content area white.
+  NSRect contentRect = [NSWindow contentRectForFrameRect:rect
+                                               styleMask:[self styleMask]];
+  [[NSBezierPath bezierPathWithRect:contentRect] addClip];
+  [[NSColor whiteColor] set];
+  NSRectFill(rect);
+}
+
 @end
 
 @interface ShellCustomFrameNSWindow : ShellNSWindow {