Have Document::elementSheet() return a reference

Have Document::elementSheet() return a reference instead of a pointer as it can
never return null. This makes the calling code look safer.

R=adamk

Review URL: https://codereview.chromium.org/181663002

git-svn-id: svn://svn.chromium.org/blink/trunk@167921 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Have Document::topDocument() return a reference

Have Document::topDocument() return a reference instead of a pointer as it can
never return null. This makes the calling code look safer.

R=adamk

Review URL: https://codereview.chromium.org/179163004

git-svn-id: svn://svn.chromium.org/blink/trunk@167922 bbb929c8-8fbe-4397-9dbb-9b2b20218538