[Oilpan]: Move Source/heap directory to Source/platform/heap and merge it into the blink_platform target.

I kept the blink_heap_unittests target and merged the old blink_heap target into the blink_platform target.
I also kept the separate blink_heap.gypi file since it seemed a bit more modular.

R=abarth@chromium.org, ager@chromium.org, erik.corry@gmail.com, haraken@chromium.org, jochen@chromium.org, oilpan-reviews@chromium.org, tkent@chromium.org, vegorov@chromium.org, zerny@chromium.org
BUG=
NOTRY=true

Review URL: https://codereview.chromium.org/221913002

git-svn-id: svn://svn.chromium.org/blink/trunk@170773 bbb929c8-8fbe-4397-9dbb-9b2b20218538