Make standalone webview crash dump percentage contingent on channel

We currently submit a manual CL every time we want to change the
hardcoded crash dump percentage for standalone webview due to releasing
a beta apk or promoting a beta to stable.

Now that standalone webview has channels
(https://bugs.chromium.org/p/chromium/issues/detail?id=919216), add
logic to include different crash reporting rates for different channels.

Bug: 954298
Change-Id: I3a6a9e32e67260cdb2190ea5b394c514d6df0c06
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1574362
Reviewed-by: Tao Bai <michaelbai@chromium.org>
Commit-Queue: Drew Stonebraker <stonebraker@chromium.org>
Cr-Commit-Position: refs/heads/master@{#652426}