Remove card elevation in Tab Grid

The elevation is rendered slightly later than the View, and this
causes flickering when entering the Grid Tab Switcher (GTS), or
switching between normal/incognito mode in GTS.

The rendering of elevation is hardware-accelerated, and cannot be
captured in View.draw(Canvas). Removing the elevation eliminates
this discrepancy, and make it possible to have a smoother transition
from the captured Bitmap to the real Android View. We'll need this
when making the Tab to GTS transition animation.

Bug: 964406
Change-Id: I725f12a8dd186d4e6f848ef7bdef9a1cc8b9556d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1617213
Commit-Queue: Wei-Yin Chen (陳威尹) <wychen@chromium.org>
Reviewed-by: Yusuf Ozuysal <yusufo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#661165}

chrome/android/features/tab_ui/java/res/layout/tab_grid_card_item.xml

@@ -13,11 +13,10 @@
         android:id="@+id/background_view"
         android:background="@drawable/popup_bg"
         android:layout_margin="3dp"
-        android:visibility="gone"/>
+        android:visibility="visible"/>
     <RelativeLayout
         android:layout_width="match_parent"
         android:layout_height="match_parent"
-        android:elevation="4dp"
         android:layout_gravity="center"
         android:background="@drawable/tab_grid_card_background"
         android:layout_margin="8dp">
@@ -72,7 +71,6 @@
         android:id="@+id/close_button"
         android:layout_width="48dp"
         android:layout_height="48dp"
-        android:elevation="4dp"
         android:scaleType="center"
         android:layout_gravity="end"
         android:tint="@color/modern_grey_800"

chrome/android/features/tab_ui/java/src/org/chromium/chrome/browser/tasks/tab_management/TabGridViewBinder.java

@@ -51,7 +51,6 @@ class TabGridViewBinder {
                         (int) res.getDimension(R.dimen.tab_list_selected_inset_kitkat));
                 Drawable elevationDrawable =
                         ResourcesCompat.getDrawable(res, R.drawable.popup_bg, theme);
-                holder.backgroundView.setVisibility(View.VISIBLE);
                 holder.backgroundView.setBackground(
                         item.get(TabProperties.IS_SELECTED) ? selectedDrawable : elevationDrawable);
             } else {