[iOS] GradientView layer is not initialized

The -[GradientView updateColors] needs to be called at least once, to
initialize correctly the gradient colors.

Before:
https://drive.google.com/open?id=1WJnG8ahbljpr0hSR8fut8ezGyIJ1i9s6
After:
https://drive.google.com/open?id=1y0xTlrpu3ssvrlw7Kx05d5lqK56a1-Ko
(the blue background was set just for the screenshots)

Bug: 971989
Change-Id: I08c1fe12f5b142fde038da9b779a7399f1771f3b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2133937
Commit-Queue: Jérôme Lebel <jlebel@chromium.org>
Auto-Submit: Jérôme Lebel <jlebel@chromium.org>
Reviewed-by: Nohemi Fernandez <fernandex@chromium.org>
Cr-Commit-Position: refs/heads/master@{#757032}

ios/chrome/browser/ui/authentication/signin/user_signin/gradient_view.mm

@@ -23,6 +23,7 @@
   self = [super initWithFrame:CGRectZero];
   if (self) {
     self.userInteractionEnabled = NO;
+    [self updateColors];
   }
   return self;
 }
@@ -31,8 +32,7 @@
   return base::mac::ObjCCastStrict<CAGradientLayer>(self.layer);
 }
 
-- (void)traitCollectionDidChange:
-    (nullable UITraitCollection*)previousTraitCollection {
+- (void)traitCollectionDidChange:(UITraitCollection*)previousTraitCollection {
   [super traitCollectionDidChange:previousTraitCollection];
   if (@available(iOS 13, *)) {
     if ([self.traitCollection
@@ -54,7 +54,7 @@
 
   self.gradientLayer.colors = @[
     (id)[UIColor.cr_systemBackgroundColor colorWithAlphaComponent:0].CGColor,
-    (id)UIColor.cr_systemBackgroundColor.CGColor
+    (id)UIColor.cr_systemBackgroundColor.CGColor,
   ];
   [CATransaction commit];
 }