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PersistentHeapCollection was removed from the code base. This removes support from blink_gc_plugin.

Bug: None
Change-Id: I7adfa680843f12a3fbd3040b7fae95c7826c9fe8
Reviewed-on: https://chromium-review.googlesource.com/1245082Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Keishi Hattori <keishi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#594285}