Oilpan: fix build after r198049.

Move the script-exposed MediaSession to the heap.

TBR=oilpan-reviews
BUG=497735
NOTRY=true

Review URL: https://codereview.chromium.org/1211973007

git-svn-id: svn://svn.chromium.org/blink/trunk@198054 bbb929c8-8fbe-4397-9dbb-9b2b20218538
3 files changed