Use black system nav bar for web apps and web apks

BUG=805976

Change-Id: Icdaf76079b16f7664c5dde3b63f429ae5473532f
Reviewed-on: https://chromium-review.googlesource.com/893647Reviewed-by: Ted Choc <tedchoc@chromium.org>
Commit-Queue: Theresa <twellington@chromium.org>
Cr-Commit-Position: refs/heads/master@{#532999}

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

@@ -98,7 +98,9 @@
     </style>
 
     <!-- Web app themes -->
-    <style name="WebappTheme" parent="MainTheme">
+    <!-- Extend MainThemeBase rather than MainTheme to avoid values-v27 navigation bar colors from
+         being applied -->
+    <style name="WebappTheme" parent="MainThemeBase">
         <item name="android:windowBackground">@null</item>
         <item name="android:windowDisablePreview">true</item>
     </style>