Fix new tab variation UI misbhavior

* Adjust the width of the '+' button in new tab variation from 48dp to
56dp.
* Properly tint the '+' button for light/dark mode.

Bug: 1042606, 1042015
Change-Id: I47a29b9d29ab7a5d719ec183c4c88de240eb399e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2005549
Auto-Submit: Yue Zhang <yuezhanggg@chromium.org>
Reviewed-by: Theresa  <twellington@chromium.org>
Commit-Queue: Yue Zhang <yuezhanggg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#732582}

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

@@ -19,12 +19,13 @@
         android:orientation="horizontal"
         android:visibility="gone">
         <ImageView
-            android:layout_width="@dimen/toolbar_button_width"
+            android:layout_width="wrap_content"
             android:layout_height="56dp"
             android:layout_gravity="start|top"
             android:scaleType="center"
             android:paddingStart="16dp"
             android:paddingEnd="16dp"
+            android:tint="@color/default_icon_color"
             app:srcCompat="@drawable/new_tab_icon"
             android:contentDescription="@string/accessibility_toolbar_btn_new_tab"/>
         <TextView