Oilpan: Replace blink_gc_plugin_dump_graph with a more general blink_gc_plugin_flags GYP variable.

This allows developers to pass any combination of plugin flags
without modifying the existing GYP setup. For example,
GYP_DEFINES+="blink_gc_plugin_flags='dump-graph=1 warn-raw-ptr=1'"
will enable both graph dumps and raw-pointer warnings.

R=haraken@chromium.org
BUG=334149

Review URL: https://codereview.chromium.org/551343003

git-svn-id: svn://svn.chromium.org/blink/trunk@181734 bbb929c8-8fbe-4397-9dbb-9b2b20218538