Improved srcset descriptor parsing.

I've refactored srcset's descriptor parser in order to improve readability of the srcset parser, and improve spec compliance.

Changes:
* Descriptor parsing is now split to its own function.
* Invalid descriptors are ignored, rather than invalidate the candidate.
  - That includes the viewport qualifiers, which are not yet supported, but may be used by authors.
* Some unnecessary cruft has been removed from the srcset parser.
* Added tests that make sure descriptors are treated as they should.

BUG=

Review URL: https://chromiumcodereview.appspot.com/23694049

git-svn-id: svn://svn.chromium.org/blink/trunk@158167 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Add a missing "const" for constructor parameter.

TBR=abarth

Review URL: https://chromiumcodereview.appspot.com/24246017

git-svn-id: svn://svn.chromium.org/blink/trunk@158168 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Drop Promise.fulfill and Promise.some static functions

Since they are removed from the consensus of Promises[1], we drop these
implementations.

[1]: https://github.com/domenic/promises-unwrapping

BUG=295420

Review URL: https://chromiumcodereview.appspot.com/24319002

git-svn-id: svn://svn.chromium.org/blink/trunk@158169 bbb929c8-8fbe-4397-9dbb-9b2b20218538