Oilpan: Revert r169564: Disable the GC_PLUGIN_IGNORE annotation when compiling with asan.

A fixed clang/asan has been rolled.
See https://code.google.com/p/address-sanitizer/issues/detail?id=279

R=ager@chromium.org
BUG=

Review URL: https://codereview.chromium.org/257403002

git-svn-id: svn://svn.chromium.org/blink/trunk@172972 bbb929c8-8fbe-4397-9dbb-9b2b20218538