Remove unneeded mutable keyword in some member variables

Some member variables have mutable keyword though they don't used by const function.
This cl removes them.

BUG=N/A

Review URL: https://codereview.chromium.org/212183004

git-svn-id: svn://svn.chromium.org/blink/trunk@170126 bbb929c8-8fbe-4397-9dbb-9b2b20218538