Fix flaky HTMLMeterElement tests.

Adding missing specialized NeedsTracing trait for CSSGradientColorStop to ensure that it is traced when part of a collection.

R=ager@chromium.org, erik.corry@gmail.com, haraken@chromium.org, oilpan-reviews@chromium.org, tkent@chromium.org, vegorov@chromium.org, zerny@chromium.org
BUG=341815

Review URL: https://codereview.chromium.org/179523002

git-svn-id: svn://svn.chromium.org/blink/trunk@167923 bbb929c8-8fbe-4397-9dbb-9b2b20218538