Notifications: restrict button icon to 32dp.

This size is as recommended in the addAction docs:
http://goo.gl/ajjlE0

BUG=581336

Review URL: https://codereview.chromium.org/1643493002

Cr-Commit-Position: refs/heads/master@{#371828}

chrome/android/java/res/layout/web_notification_button.xml

@@ -22,10 +22,12 @@
     <ImageView
         android:id="@+id/button_icon"
         android:layout_width="wrap_content"
-        android:layout_height="match_parent"
+        android:layout_height="wrap_content"
         android:layout_marginStart="8dp"
         android:layout_gravity="start|center_vertical"
-        android:maxWidth="48dp"
+        android:adjustViewBounds="true"
+        android:maxHeight="32dp"
+        android:maxWidth="32dp"
         android:contentDescription="@null"
         android:scaleType="centerInside"/>