[Touchless] Give last tab button a width independent of contents

Change the button from wrap_content to a width relative to the width
of parent element.

Bug: 936052
Change-Id: I63ff28b1329a221255f14113f9ae65dba2cee514
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1575361
Commit-Queue: Brandon Wylie <wylieb@chromium.org>
Reviewed-by: Justin DeWitt <dewittj@chromium.org>
Cr-Commit-Position: refs/heads/master@{#652896}

chrome/android/touchless/java/res/layout/new_tab_page_touchless.xml

@@ -16,7 +16,7 @@
 
     <org.chromium.chrome.browser.touchless.OpenLastTabView
         android:id="@+id/open_last_tab_button"
-        android:layout_width="wrap_content"
+        android:layout_width="match_parent"
         android:layout_height="wrap_content" />
 
     <ViewStub android:id="@+id/most_likely_stub"