Motivation: There are a few things we want to do in Skia infrastructure that is not possible to do without this functionality.
Eg1: If there is a change made to Skia's markdown then automatically include a 'DOCS_PREVIEW_URL=https://skia.org?cl=1234' in the CL's description.
Eg2: Automatically add 'NOTRY=true' for changes impacting things that do not need trybot runs.
Eg3: Include CL specific links to skia's perf (https://perf.skia.org/) and correctness (https://gold.skia.org/) servers.

BUG=chromium:462208

Review URL: https://codereview.chromium.org/949273002

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@294242 0039d316-1c4b-4281-b951-d872f2087c98