Make test262 README file revision independent.

This makes the description in the README file for our test262 harness
independent of the underlying revision. The one canonical place to
mention the up-to-date revision is the DEPS file.

R=machenbach@chromium.org
NOTRY=true

Review URL: https://codereview.chromium.org/1424043006

Cr-Commit-Position: refs/heads/master@{#31778}

test/test262/README

@@ -3,12 +3,12 @@ into the v8 test harness. The tests are checked out as a dependency from
 
   https://chromium.googlesource.com/external/github.com/tc39/test262
 
-at hash 26e6fd7 (2015/10/01 revision) as 'data' in this directory. They are
+at a revision specified in the DEPS file as 'data' in this directory. They are
 fetched with 'gclient sync'. To update to a newer version, please change the
 DEPS file.
 
 Using a newer version, e.g. 'deadbeef' may be possible but the tests are only
-known to pass (and indeed run) with that revision. Example:
+known to pass (and indeed run) with the DEPS revision. Example:
 
   cd data
   git fetch