Revert of Update V8 to version 3.28.2 (based on bleeding_edge revision r21987).

DEPS

 # This file is automatically processed to create .DEPS.git which is the file
 # that gclient uses under git.
 #
 # See http://code.google.com/p/chromium/wiki/UsingGit
 #
 # To test manually, run:
 #   python tools/deps2git/deps2git.py -o .DEPS.git -w <gclientdir>
 # where <gcliendir> is the absolute path to the directory containing the
 # .gclient file (the parent of "src").
 #
@@ skipping 32 matching lines @@
   "libphonenumber_revision": "621",
   "libvpx_revision": "278986",
   "lss_revision": "26",
   "sfntly_revision": "239",
   "lighttpd_revision": "33737",
   "skia_revision": "474cef4c55818bcf6ef1f7227417833ec82c5a36",
   # Three lines of non-changing comments so that
   # the commit queue can handle CLs rolling Skia
   # and V8 without interference from each other.
   "v8_branch": "trunk",
-  "v8_revision": "21989",
+  "v8_revision": "21978",
   # Three lines of non-changing comments so that
   # the commit queue can handle CLs rolling WebRTC
   # and V8 without interference from each other.
   "webrtc_revision": "6496",
   "jsoncpp_revision": "248",
   "nss_revision": "277057",
   # Three lines of non-changing comments so that
   # the commit queue can handle CLs rolling swarming_client
   # and whatever else without interference from each other.
   "swarming_revision": "c333224ef8cf5cc41ab91f6ee9cb1fdbe3689f19",
@@ skipping 738 matching lines @@
                 "src/third_party/apache-win32",
     ],
   },
   {
     # A change to a .gyp, .gypi, or to GYP itself should run the generator.
     "name": "gyp",
     "pattern": ".",
     "action": ["python", "src/build/gyp_chromium"],
   },
 ]