Fill in descriptions on relevant blink_perf tests

The new descriptions make it easier to quickly determine what each test is
doing. There should be no performance impact on the tests themselves as
setting the description is not in the critical section of the tests.

BUG=404274

Review URL: https://codereview.chromium.org/478003002

git-svn-id: svn://svn.chromium.org/blink/trunk@184155 bbb929c8-8fbe-4397-9dbb-9b2b20218538