Oilpan: Fix document access in HTMLMediaElement destructor.

After investigating complicated solutions to getting the load delay
count decremented correctly I realized that we have a weak map
from Document to the HTMLMediaElements for that Document. Therefore,
in the HTMLMediaElement destructor we know if we are dying with the
Document or not. If we are dying with the document, the weak mapping
has been cleared during weak processing. If we are dying and the
document is still alive, the document will still be in the mapping
and will be alive so it is safe to access it.

R=erik.corry@gmail.com, oilpan-reviews@chromium.org, sigbjornf@opera.com
BUG=374691

Review URL: https://codereview.chromium.org/294933002

git-svn-id: svn://svn.chromium.org/blink/trunk@174373 bbb929c8-8fbe-4397-9dbb-9b2b20218538