Set android:colorTheme in Chrome for Android's MainTheme

I am working on re-implementing the Android text edit widget's spell check menu
in Chrome and the Android WebView. The buttons in the native Android spell check
menu use the value of android:colorAccent set by the embedding app. To make the
WebView spell check menu have the same behavior, we need to also make it use
android:colorAccent. But to make the menu use the correct (according to
hannahs@, our UX designer) blue color, and not the default green color, we need
to set colorAccent in our MainTheme. This change does that.

I'm assuming that we don't currently have anything that is relying on this being
the existing green color (since that wouldn't match the color our designer said
we're supposed to use for these UI elements anyway). We do currently have some
themes (e.g. FullscreenWhite and PreferencesDialogTheme) that are already
setting this color to other values for specific parts of the app though.

Note: I've tested this change on Jelly Bean and the app seems to run OK, despite
android:colorAccent having been added in a later API version.

Bug: 715365
Change-Id: I2690c014d18211304ace6894f5a2cea0db72a1af
Reviewed-on: https://chromium-review.googlesource.com/566031Reviewed-by: Theresa <twellington@chromium.org>
Commit-Queue: Ryan Landay <rlanday@chromium.org>
Cr-Commit-Position: refs/heads/master@{#485486}

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

@@ -23,6 +23,7 @@
         <item name="android:textColorHighlight">@color/text_highlight_color</item>
         <item name="android:textColorLink">@color/light_active_color</item>
         <item name="colorPrimaryDark">@android:color/black</item>
+        <item name="android:colorAccent">@color/google_blue_500</item>
         <item name="android:statusBarColor" tools:targetApi="21">@android:color/black</item>
 
         <!--  Overriding AppCompat values -->