Cleanup tests related to controls hiding/fading in/out

Add a helper function runAfterControlsHidden() that computes a reasonable
timeout for when the controls should be invisible.
Add controlsFade{In,Out}DurationMs to avoid having to redefine them in
every test that needs them.
Convert tests to use the helper and the variables, and also perform some
opportunity simplifications to lookups of the panel shadow element.

Review URL: https://codereview.chromium.org/302603003

git-svn-id: svn://svn.chromium.org/blink/trunk@175365 bbb929c8-8fbe-4397-9dbb-9b2b20218538