[heap, blink_gc_plugin] Add option to disable collections check

This CL temporarily disables the check by default, adds an option to
disable it via flags, and removes the built in pdfium exemption.
The check remains enabled for blink via the BUILD.gn file.
After this CL rolls, we can add the disable flag to Pdfium's BUILD.gn
and re-enable the check by default.

Bug: chromium:1470687
Change-Id: Ibe308408e629195be2a6bb13c31a957ad06972e7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5093797
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Omer Katz <omerkatz@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1233888}