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Currently safebrowsing is crashing due to the lack
of pref service in weblayer (see crbug.com/1025257).
This patch creates and initializes the pref service to
avoid this crash.

BUG=1015418,1025257
TBR=gab@chromium.org

Change-Id: I11e2ab3bc6cebbd973ad816152dfc27a090c5124
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1926321
Commit-Queue: Tim Volodine <timvolodine@chromium.org>
Reviewed-by: Richard Coles <torne@chromium.org>
Cr-Commit-Position: refs/heads/master@{#717770}