Set default text of the search widget to "Search"

This removes the dependency on an immediate onUpdate from the framework
to have a default title.

BUG=

Change-Id: I0e36b40b25e0a26c618e48cbf99c4a470f19ccf0
Reviewed-on: https://chromium-review.googlesource.com/918781Reviewed-by: Yusuf Ozuysal <yusufo@chromium.org>
Commit-Queue: Ted Choc <tedchoc@chromium.org>
Cr-Commit-Position: refs/heads/master@{#536731}

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

@@ -31,6 +31,7 @@
             android:gravity="center_vertical"
             android:singleLine="true"
             android:textAlignment="viewStart"
+            android:text="@string/search_widget_default"
             android:textColor="@color/google_grey_600"
             android:textSize="17sp" />