Add flag for SCT auditing

Adds an entry to chrome://flags for enabling SCT auditing and a set of
variations to pick from for different sampling rates (0%, 0.01%, or
0.1%). This will allow for manual testing and fishfooding without
needing to specify parameters on the command line.

Bug: 1136620
Change-Id: I3d8c0e3f6dbe5d77704b1d64dd40b5243c20182f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2462237
Reviewed-by: Mustafa Emre Acer <meacer@chromium.org>
Commit-Queue: Chris Thompson <cthomp@chromium.org>
Cr-Commit-Position: refs/heads/master@{#815369}