Oilpan: Disable the GC_PLUGIN_IGNORE annotation when compiling with asan.

For some reason having this annotation leads to linker errors on the
oilpan asan build.

obj/third_party/WebKit/Source/modules/libmodules.a(obj/third_party/WebKit/Source/modules/webdatabase/modules.DatabaseSync.o):../../third_party/WebKit/Source/modules/webdatabase/DatabaseSync.cpp:function __unnamed_1: error: undefined reference to '.str'

obj/third_party/WebKit/Source/modules/libmodules.a(obj/third_party/WebKit/Source/modules/webdatabase/modules.DatabaseSync.o):../../third_party/WebKit/Source/modules/webdatabase/DatabaseSync.cpp:function __unnamed_1: error: undefined reference to '.str1'

Kostya, do you have any ideas why this might break the asan build?

R=kcc@chromium.org, vegorov@chromium.org, zerny@chromium.org
BUG=

Review URL: https://codereview.chromium.org/204673002

git-svn-id: svn://svn.chromium.org/blink/trunk@169564 bbb929c8-8fbe-4397-9dbb-9b2b20218538