Mark MediaQueryData as stack allocated.

Its only use is as a part object of the stack-allocated MediaQueryParser.

R=ager@chromium.org
BUG=
NOTRY=true

Review URL: https://codereview.chromium.org/207743002

git-svn-id: svn://svn.chromium.org/blink/trunk@169725 bbb929c8-8fbe-4397-9dbb-9b2b20218538