back gesture nudge: Increase the gradient layer width.

As requested by the UX.

Bug: 1009005b
Change-Id: I3c175dbbb8074c47400df3777ab849dc313294f0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2086072Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Commit-Queue: Xiaoqian Dai <xdai@chromium.org>
Cr-Commit-Position: refs/heads/master@{#746829}

ash/wm/gestures/back_gesture/back_gesture_contextual_nudge.cc

@@ -23,7 +23,7 @@ namespace ash {
 namespace {
 
 // Width of the contextual nudge.
-constexpr int kBackgroundWidth = 160;
+constexpr int kBackgroundWidth = 320;
 
 // Colors of the contextual nudge background gradient.
 constexpr SkColor kBackgroundStartColor =