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This reverts https://codereview.chromium.org/1129103005/ , which turned
them off.  Now what all [blink-gc] warnings are fixed, we can turn them
on again on the warnings-as-errors 64-bit builds.

BUG=486571,467287
TBR=hans

Review URL: https://codereview.chromium.org/1131333003

Cr-Commit-Position: refs/heads/master@{#330313}