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This can be used to replace the current default ComponentContext with
one backed by a supplied |service_root| channel, e.g. for testing.

Bug: 893229
Change-Id: Ib08caf7527066c5b269d756a1726e1843c6431f5
Reviewed-on: https://chromium-review.googlesource.com/c/1333113
Commit-Queue: Wez <wez@chromium.org>
Reviewed-by: Sergey Ulanov <sergeyu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#607788}