Android: Fix gesture navigation bubble UI shape

This CL reverts a change made about gesture navigation navigation
bubble dimension in https://crrev.com/c/1650205. Since it is not a
touch target, doesn't have to be as big as explained in the CL for
a11y.

Bug: 974633
Change-Id: I5b1a36b6c2d166f0964caae039f98f67458f6c5b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1663228Reviewed-by: Becky Zhou <huayinz@chromium.org>
Commit-Queue: Jinsuk Kim <jinsukkim@chromium.org>
Cr-Commit-Position: refs/heads/master@{#669890}

chrome/android/java/res/values/dimens.xml

@@ -603,8 +603,8 @@
 
     <!-- History Navigation UI Item -->
     <dimen name="navigation_bubble_border_width">1dp</dimen>
-    <dimen name="navigation_bubble_default_height">48dp</dimen>
-    <dimen name="navigation_bubble_bg_vertical_inset">8dp</dimen>
+    <dimen name="navigation_bubble_default_height">32dp</dimen>
+    <dimen name="navigation_bubble_bg_vertical_inset">0dp</dimen>
     <dimen name="navigation_bubble_vertical_padding">4dp</dimen>
     <dimen name="navigation_bubble_horizontal_padding">8dp</dimen>
     <dimen name="navigation_bubble_icon_size">20dp</dimen>