Pull the list of virtual test suites from LayoutTests/VirtualTestSuites.

Previously the list of suites was hardcoded in webkitpy/layout_tests/port/base.py,
which meant that developers had to modify code when adding or removing suites in LayoutTests/.

The new file is required to be in (strict) JSON format, and must contain a list of objects, one 
per test suite. Each object should have three keys:

  - "base": the underlying directory of tests to run
  - "args": the additional command line arguments to use
  - one of "prefix" or "name", indicating the path to use for new baselines and
    test expectations. 
     - If "prefix" is used (preferred), the resulting full path is "LayoutTests/virtual/{prefix}/{base}/{test}"
     - If "name" is used, the resulting full path is "LayoutTests/virtual/{name}/{test}". This is used only
        for "legacy" test suites.

    There are only two legacy suites that use "name", and they will be migrated to "prefix" shortly.
        
R=ojan@chromium.org
BUG=410120

Review URL: https://codereview.chromium.org/530323002

git-svn-id: svn://svn.chromium.org/blink/trunk@181420 bbb929c8-8fbe-4397-9dbb-9b2b20218538