[variations] Enable sticky activation on Canary.

This implements VariationsServiceClient::IsStickyActivationEnabled()
for Chrome, iOS, and WebView.

The function returns true if the current channel is UNKNOWN or
CANARY, enabling studies with sticky activation on these channels.
(Unknown channel can be the case in tests.)

The method ordering in the header and implementation files has been
made consistent for the modified classes.

This has the effect of the new StickyActivationManager (implemented
under crbug.com/435630455) of running on canary and studies with
the STICKY_AFTER_QUERY activation mode being handled. Once this
is out on canary, I plan on exercising these changes with a test study.

The extra channel-gating is a safety precaution to ensure the
new functionality doesn't get widely rolled out before it has
been sufficiently tested in the wild.

Bug: 435630455
Change-Id: I7d300e026d7e15ca807f6015c0056472ba9e9ae2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6828554
Reviewed-by: Nate Fischer <ntfschr@chromium.org>
Auto-Submit: Alexei Svitkine <asvitkine@chromium.org>
Commit-Queue: Alexei Svitkine <asvitkine@chromium.org>
Commit-Queue: Nate Fischer <ntfschr@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1499051}