[ios] Give the root view controller a white background.

The root view controller is briefly visible during cold start, so give it a
white background to make it look more similar to the splash screen.  This helps
avoid a noticeable flicker of black at startup.

BUG=788746

Cq-Include-Trybots: master.tryserver.chromium.mac:ios-simulator-cronet;master.tryserver.chromium.mac:ios-simulator-full-configs
Change-Id: Ic107c831a122caece7efc709bcde008a8ba7edcc
Reviewed-on: https://chromium-review.googlesource.com/791158
Commit-Queue: Rohit Rao <rohitrao@chromium.org>
Reviewed-by: Mark Cogan <marq@chromium.org>
Cr-Commit-Position: refs/heads/master@{#519702}

ios/chrome/browser/ui/main/main_presenting_view_controller.mm

@@ -105,6 +105,11 @@
 @synthesize tabSwitcher = _tabSwitcher;
 @synthesize bvcContainer = _bvcContainer;
 
+- (void)viewDidLoad {
+  // Set a white background color to avoid flickers of black during startup.
+  self.view.backgroundColor = [UIColor whiteColor];
+}
+
 - (UIViewController*)activeViewController {
   if (self.bvcContainer) {
     DCHECK_EQ(self.bvcContainer, self.presentedViewController);