Use center instead of center_vertical|center_horizontal

The layout_gravity value center does the same thing as
center_vertical|center_horizontal. This CL makes use of center.

Change-Id: I625ffe5dc5b147ed39d986d862f38ee432fbfa7a
Reviewed-on: https://chromium-review.googlesource.com/1102909Reviewed-by: Ted Choc <tedchoc@chromium.org>
Commit-Queue: Pedro Amaral <amaralp@chromium.org>
Cr-Commit-Position: refs/heads/master@{#567779}

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

@@ -43,7 +43,7 @@
                 android:id="@+id/home_button"
                 style="@style/ToolbarButton"
                 android:src="@drawable/btn_toolbar_home"
-                android:layout_gravity="center_vertical|center_horizontal"
+                android:layout_gravity="center"
                 android:contentDescription="@string/accessibility_toolbar_btn_home"/>
 
             <Space
@@ -55,7 +55,7 @@
                 android:id="@+id/search_button"
                 style="@style/ToolbarMenuButtonPhone"
                 android:src="@drawable/ic_logo_googleg_24dp"
-                android:layout_gravity="center_vertical|center_horizontal"
+                android:layout_gravity="center"
                 android:contentDescription="@string/accessibility_toolbar_btn_menu" />
 
             <Space
@@ -66,7 +66,7 @@
             <org.chromium.chrome.browser.toolbar.TabSwitcherButtonView
                 android:id="@+id/tab_switcher_button"
                 style="@style/ToolbarButton"
-                android:layout_gravity="center_vertical|center_horizontal"
+                android:layout_gravity="center"
                 android:contentDescription="@string/accessibility_toolbar_btn_tabswitcher_toggle_default" />
 
             <Space
@@ -78,7 +78,7 @@
                 android:id="@+id/menu_button"
                 style="@style/ToolbarMenuButtonPhone"
                 android:src="@drawable/ic_more_vert_black_24dp"
-                android:layout_gravity="center_vertical|center_horizontal"
+                android:layout_gravity="center"
                 android:contentDescription="@string/accessibility_toolbar_btn_menu"
                 app:chrometint="@color/dark_mode_tint" />