Chrome OS OOBE: make icons of the Back and Next button use button color.

This Cl enables "Back" and "Forward" icons to follow the color style of the
button.

Bug: 852551
Change-Id: I75cec2058d7e330db5c7b5d226f47cd0f9f54571
Reviewed-on: https://chromium-review.googlesource.com/1171956Reviewed-by: Wenzhao (Colin) Zang <wzang@chromium.org>
Commit-Queue: Alexander Alekseev <alemate@chromium.org>
Cr-Commit-Position: refs/heads/master@{#582473}

chrome/browser/resources/chromeos/login/oobe_buttons.html

@@ -14,9 +14,8 @@
 <iron-iconset-svg name="oobe-buttons-20" size="20">
   <svg>
     <defs>
-      <g id="arrow-forward" fill="none" fill-rule="evenodd">
-        <path d="M0 0h20v20H0z"></path>
-        <path fill="#FFF" fill-rule="nonzero" d="M7 13.825L11.327 10 7 6.175 8.332 5 14 10l-5.668 5z">
+      <g id="arrow-forward" fill-rule="nonzero">
+        <path d="M7 13.825L11.327 10 7 6.175 8.332 5 14 10l-5.668 5z">
         </path>
       </g>
     </defs>
@@ -26,9 +25,8 @@
 <iron-iconset-svg name="oobe-buttons-40" size="40">
   <svg>
     <defs>
-      <g id="arrow-forward" fill="none" fill-rule="evenodd">
-        <path d="M0 0h40v40H0z"></path>
-        <path fill="#FFF" fill-rule="nonzero" d="M14 27.65L22.653 20 14 12.35 16.664 10 28 20 16.664 30z">
+      <g id="arrow-forward" fill-rule="nonzero">
+        <path d="M14 27.65L22.653 20 14 12.35 16.664 10 28 20 16.664 30z">
         </path>
       </g>
     </defs>