Have Element::ensureMutableInlineStyle() return a reference

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

Also update StyleRule*::properties() / StyleRule*::mutableProperties() to return
a reference as a result. This gets rid of several unnecessary null checks as
well.

R=adamk

Review URL: https://codereview.chromium.org/181783005

git-svn-id: svn://svn.chromium.org/blink/trunk@168044 bbb929c8-8fbe-4397-9dbb-9b2b20218538