Enable OffMainThreadFetch by default

We did the field trial experiment in Dev/Canary 10 days.
And we haven't received related crashes.
So we decided to enable this feature by default.

Bug:752857, 443374

Cq-Include-Trybots: master.tryserver.chromium.linux:linux_layout_tests_slimming_paint_v2
Change-Id: I223d52e69ad826297ea2359b6506e2a62b4b6e18
Reviewed-on: https://chromium-review.googlesource.com/567845
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Commit-Queue: Tsuyoshi Horo <horo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#495097}
56 files changed