Add comments to vectors/hash sets of raw pointers to on-heap objects used in webaudio/

webaudio/ sometimes has to use vectors/hash sets of raw pointers to on-heap objects,
if items are added to those vectors/hash sets by the audio thread. This is because
the audio thread is not registered to Oilpan and thus the audio thread is not allowed
to expand entries of the vectors/hash sets.

This CL adds comments to those vectors/hash sets about why they use off-heap
vectors/hash sets of raw pointers.

BUG=340522
NOTRY=true

Review URL: https://codereview.chromium.org/445453006

git-svn-id: svn://svn.chromium.org/blink/trunk@179552 bbb929c8-8fbe-4397-9dbb-9b2b20218538