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This flag is now enabled by default and no longer necessary, so this CL
removes it.

We could remove more code (e.g., PrerenderManager::SetMode()), but this
CL focuses on the flag to minimize changes. Subsequent CLs will remove
them.

Bug: 632361
Change-Id: Ib4a0edfbfb4c62c6e98a112ecf7fdfa604063454
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2485412
Commit-Queue: Hiroki Nakagawa <nhiroki@chromium.org>
Reviewed-by: Robert Ogden <robertogden@chromium.org>
Reviewed-by: Ryan Sturm <ryansturm@chromium.org>
Reviewed-by: Matt Falkenhagen <falken@chromium.org>
Reviewed-by: Angela Yoeurng <yoangela@chromium.org>
Reviewed-by: Ilya Sherman <isherman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#818823}