Remove SafeBrowsing.ThreatReport.{DomIsAmbiguous,MaxNodesExceededInFrame}

DomIsAmbiguous: This metric is false only at most 0.476% over the last 1 year.
MaxNodesExceededInFrame: This metric is true only at most %0.342 over the last 1 year.

R=lpz

Bug: 975258
Change-Id: Ic804a93f8323dda441ad1c3ffcde5c499b19fbc8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1721557
Commit-Queue: Varun Khaneja <vakh@chromium.org>
Auto-Submit: Varun Khaneja <vakh@chromium.org>
Reviewed-by: Brian White <bcwhite@chromium.org>
Reviewed-by: Luke Z <lpz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#684204}
6 files changed