DevTools: Extract drop down menu implementation based on select element into a separate ui component.

This will be needed to enhance add rule '+' button with an ability to choose stylesheet to add rule to.

R=pfeldman

Review URL: https://codereview.chromium.org/179813004

git-svn-id: svn://svn.chromium.org/blink/trunk@168015 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Deflake some layer-creation/overlap-animation tests.

TBR=shans
BUG=347086

Review URL: https://codereview.chromium.org/182973002

git-svn-id: svn://svn.chromium.org/blink/trunk@168016 bbb929c8-8fbe-4397-9dbb-9b2b20218538