Fixed srcset descriptor parsing bug and added tests

Srcset's 'w' descriptor parsing accepted descriptors of the form "400pw".
This CL fixes that by making their parsing stricter.
It also adds a bunch of tests to prevent it from regressing, and to make sure
float based descriptors are properly parsed as well.

BUG=389575

Review URL: https://codereview.chromium.org/355953003

git-svn-id: svn://svn.chromium.org/blink/trunk@177120 bbb929c8-8fbe-4397-9dbb-9b2b20218538