[DevTools] Color values should be case insensitive while suggestions should be case aware

This CL makes following changes:

* Color swatch is not shown for capitalized CSS property names. 
  As the CSS property names in caps are allowed and applied by the browser,
  added support for showing color swatch for the color values ignoring the case.

* Prompt Suggestions should be case-aware to the user input
  Added support for showing the suggestions in CSS value/name prompts honoring the user entered case.
  
BUG=392052

Review URL: https://codereview.chromium.org/376803002

git-svn-id: svn://svn.chromium.org/blink/trunk@178093 bbb929c8-8fbe-4397-9dbb-9b2b20218538