Fix accessibility issues in custom tab UI

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

Index: chrome/android/java/res/layout/custom_tabs_toolbar.xml
diff --git a/chrome/android/java/res/layout/custom_tabs_toolbar.xml b/chrome/android/java/res/layout/custom_tabs_toolbar.xml
index 5f117bee6a98987d42780a2958cc2f20215775fe..43ed0a1655149b52d1b74aa615143b04436b3697 100644
--- a/chrome/android/java/res/layout/custom_tabs_toolbar.xml
+++ b/chrome/android/java/res/layout/custom_tabs_toolbar.xml
@@ -8,7 +8,8 @@
         android:id="@+id/back_button"
         style="@style/ToolbarButton"
         android:layout_gravity="center_vertical"
-        android:src="@drawable/btn_chevron_left" />
+        android:src="@drawable/btn_chevron_left"
+        android:contentDescription="@string/accessibility_toolbar_btn_back" />

[Yusuf, 2015/06/29 18:03:17]

is this the string we want to have? Or we need a new one?

[Ian Wen, 2015/06/29 18:27:42]

This string only says "Go back". Do you want me to create a string saying
something like "Go back to previous screen" or "Go back to client app" or "close
webpage"?

[Yusuf, 2015/06/29 18:30:07]

Yes, lets add a new one and say "Close webpage"

[Ian Wen, 2015/06/30 18:31:23]

Done.

     <ImageButton android:id="@+id/security_button"
         style="@style/LocationBarButton"
         android:layout_width="@dimen/location_bar_icon_width"