Set explicit button style for snackbar action button

Snackbars are used in activities that don't have a button style set.
Explicitly set a style on the snackbar action button.

BUG=885095

Change-Id: I20bd7447475cc8da287a2a96b23f8a0f025dbcd6
Reviewed-on: https://chromium-review.googlesource.com/1232156Reviewed-by: Becky Zhou <huayinz@chromium.org>
Commit-Queue: Theresa <twellington@chromium.org>
Cr-Commit-Position: refs/heads/master@{#592226}

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

@@ -71,6 +71,7 @@
             android:textAppearance="@style/WhiteBody" />
 
         <Button
+            style="@style/MainButtonStyle"
             android:id="@+id/snackbar_button"
             android:layout_width="wrap_content"
             android:layout_height="@dimen/snackbar_min_height"