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Each time a dependency is changed, the script will now check for known
vulnerabilities. This is not perfect since ideally we'd be checking on
a regular cadence; it's likely that equivalent functionality will be
moved into Vomit or some other automated system in the future, but this
is a good interim step to ensure that a large fraction of Chrome's
open-source dependencies (212 out of 717) have some automated monitoring
for vulnerabilities, where they previously had only manual monitoring.

Testing done:
* Add this line to build.gradle and ensure fetch_all.py fails
  with the desired diagnostics.
   compile "org.jetbrains.kotlin:kotlin-stdlib:1.2.70"
* Add --ignore-vulnerabilities and ensure it continues.

Bug: 1105911
Bug: 895969
Change-Id: If017d73765ef366959595facb000fc52a528cd08
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2317706
Commit-Queue: Adrian Taylor <adetaylor@chromium.org>
Commit-Queue: Peter Wen <wnwen@chromium.org>
Reviewed-by: Andrew Grieve <agrieve@chromium.org>
Reviewed-by: Peter Wen <wnwen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#791951}