PreviewTab: Do not handle back button if suppressed

This CL changes the bottom sheet behavior so that Preview Tab in
suppressed state does not consume back button.

Bug: 1065286
Change-Id: I620fc3c74877e5af3096a635d9cb2f9c82def66f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2125908Reviewed-by: Matthew Jones <mdjones@chromium.org>
Commit-Queue: Jinsuk Kim <jinsukkim@chromium.org>
Cr-Commit-Position: refs/heads/master@{#754714}

chrome/android/java/src/org/chromium/chrome/browser/widget/bottomsheet/BottomSheetController.java

@@ -411,7 +411,9 @@ public class BottomSheetController implements Destroyable {
      * @return {@code true} if the sheet or content handled the back press.
      */
     public boolean handleBackPress() {
-        if (mBottomSheet == null) return false;
+        // If suppressed (therefore invisible), users are likely to expect for Chrome
+        // browser, not the bottom sheet, to react. Do not consume the event.
+        if (mBottomSheet == null || mIsSuppressed) return false;
 
         // Give the sheet the opportunity to handle the back press itself before falling to the
         // default "close" behavior.