Recognize GC_PLUGIN_IGNORE() on 'operator new' declarations.
Recognize GC_PLUGIN_IGNORE() on 'operator new' declarations.

For the select few times an overriding "operator new" is needed for a
GC class, provide the GC_PLUGIN_IGNORE() escape hatch for these. The
responsibility that the "new"'s allocation will be on the Oilpan heap,
rests with the implementor of the class.

R=haraken
BUG=443854

Review URL: https://codereview.chromium.org/975393002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#319141}

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