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This fixes the issue with the value set by ScopedCurrentChannel
potentially being overwritten by the browser process initialization
code.

This could happen when the ScopedCurrentChannel instance was constructed
with the default channel (that is, the STABLE channel) before the browser
initialization code happens, as the latter would then assume that the channel
wasn't set before and overwrite it.

This issue means that it was impossible to set up the STABLE channel in
the browser test in the setup code that runs before the browser initialization,
unless the building environment was configured to be stable.

BUG=none, 626343
TEST=existing tests

Review-Url: https://codereview.chromium.org/2854293003
Cr-Commit-Position: refs/heads/master@{#473402}