[Explore sites]: rename the button background style.

This is to make it clear that it is its own snowflake and should be fixed later.

Bug: 852075
Change-Id: I30991e144e6e39b433757a7aaa3749a6bb20c85a
Reviewed-on: https://chromium-review.googlesource.com/1113619Reviewed-by: Theresa <twellington@chromium.org>
Commit-Queue: Cathy Li <chili@chromium.org>
Cr-Commit-Position: refs/heads/master@{#570205}

chrome/android/java/res/drawable/button_white_border.xml → chrome/android/java/res/drawable/explore_sites_button_background.xml

@@ -6,6 +6,7 @@
      found in the LICENSE file.
 -->
 
+<!-- TODO(chili): Remove after explore sites prototype, or restyle to generally accepted style -->
 <shape xmlns:android="http://schemas.android.com/apk/res/android"
         android:shape="rectangle">
     <solid android:color="@color/light_background_color" />

chrome/android/java/res/values-v17/styles.xml

@@ -775,6 +775,6 @@
     <!-- Explore sites section -->
     <style name="ExploreSitesAction" parent="ButtonCompatBorderless">
         <item name="android:textAppearance">@style/BlueLink2</item>
-        <item name="android:background">@drawable/button_white_border</item>
+        <item name="android:background">@drawable/explore_sites_button_background</item>
     </style>
 </resources>

chrome/android/java/res/values-v21/styles.xml

@@ -120,7 +120,7 @@
     <!-- Explore sites section -->
     <style name="ExploreSitesAction" parent="ButtonCompatBorderless">
         <item name="android:textAppearance">@style/BlueLink2</item>
-        <item name="android:background">@drawable/button_white_border</item>
+        <item name="android:background">@drawable/explore_sites_button_background</item>
         <!-- API 21 introduces state list animation. This will remove the auto-shadow -->
         <item name="android:stateListAnimator">@null</item>
     </style>