Update README to reflect new paths from webtry repo move

BUG=skia:
TBR=jcgregorio

Review URL: https://codereview.chromium.org/692613003
Adapting scripts of skiaperf for running skiacorrectness on the same server.

BUG=skia:

Review URL: https://codereview.chromium.org/671183004
Polymerize the button that toggles its sibling.

BUG=skia:

Review URL: https://codereview.chromium.org/689683003
Change the signatures of the DiffStore methods.

Allow an array of digests to be passed in because:
* stephana@ suggested that it will make things a little simpler for callers.
* This is a stepping stone for adding parallelism in filediffstore, which will
  help bring down the time taken to copy from Google Storage and generate diffs.

Also, added a unit test for AbsPath.

BUG=skia:2930

Review URL: https://codereview.chromium.org/690743002