Restore clip coalescing for multiple normal box-shadows.

A similar change was previously committed as r151971, but was reverted
(inadvertently) as part of another change.

BUG=244419

Review URL: https://chromiumcodereview.appspot.com/22923021

git-svn-id: svn://svn.chromium.org/blink/trunk@156440 bbb929c8-8fbe-4397-9dbb-9b2b20218538