Avoid WeakProcessingHashTableHelper<> type redefinitions.
Avoid WeakProcessingHashTableHelper<> type redefinitions.

R=haraken
BUG=

Review URL: https://codereview.chromium.org/1770103003

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#379820}

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