Oilpan: Added gyp flag to dump points-to graph information from the Blink GC plugin.

Enabling the blink_gc_plugin_dump_graph flag will cause the Blink GC plugin for clang to generate files with points-to information for each compilation unit. The output files can be used to statically analyze the complete points-to graph for cycles and other correctness criteria.

R=ager@chromium.org
BUG=

Review URL: https://codereview.chromium.org/207393003

git-svn-id: svn://svn.chromium.org/blink/trunk@170142 bbb929c8-8fbe-4397-9dbb-9b2b20218538