[Explore sites]: Add more horizontal spacing to tiles.

Bug: 894927,867488
Change-Id: I8c92aa6b3561aed2efd6332a0b1cd3ecde8cdb9b
Reviewed-on: https://chromium-review.googlesource.com/c/1277695Reviewed-by: Theresa <twellington@chromium.org>
Commit-Queue: Cathy Li <chili@chromium.org>
Cr-Commit-Position: refs/heads/master@{#599339}

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

@@ -7,6 +7,7 @@
     xmlns:android="http://schemas.android.com/apk/res/android"
     android:layout_width="@dimen/tile_view_width"
     android:layout_height="wrap_content"
+    android:layout_marginHorizontal="@dimen/explore_sites_site_tile_spacing"
     android:minHeight="@dimen/explore_sites_site_tile_min_height"
     android:paddingTop="@dimen/explore_sites_site_tile_padding" >
     <include

chrome/android/java/res/values/dimens.xml

@@ -357,6 +357,7 @@
     <dimen name="explore_sites_page_padding">12dp</dimen>
     <dimen name="explore_sites_site_tile_min_height">88dp</dimen>
     <dimen name="explore_sites_site_tile_padding">16dp</dimen>
+    <dimen name="explore_sites_site_tile_spacing">6dp</dimen>
 
     <!-- Recent tabs page -->
     <dimen name="recent_tabs_visible_separator_padding">8dp</dimen>