Update V8 to version 5.5.81.

DEPS

 # This file is used to manage the dependencies of the Chromium src repo. It is
 # used by gclient to determine what version of each dependency to check out, and
 # where.
 #
 # For more information, please refer to the official documentation:
 #   https://sites.google.com/a/chromium.org/dev/developers/how-tos/get-the-code
 #
 # When adding a new dependency, please update the top-level .gitignore file
 # to list the dependency's destination directory.
 #
@@ skipping 22 matching lines @@
   # the commit queue can handle CLs rolling sfntly
   # and whatever else without interference from each other.
   'sfntly_revision': 'b18b09b6114b9b7fe6fc2f96d8b15e8a72f66916',
   # Three lines of non-changing comments so that
   # the commit queue can handle CLs rolling Skia
   # and whatever else without interference from each other.
   'skia_revision': '443c5843aa700e507316d069fd6610ab675f7794',
   # Three lines of non-changing comments so that
   # the commit queue can handle CLs rolling V8
   # and whatever else without interference from each other.
-  'v8_revision': '0e51f51f634aa2044cd6121d746d0d8dd6fe4528',
+  'v8_revision': '313998549e50055c4c68a86c61710b73b1954654',
   # 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': '380e32662312eb107f06fcba6409b0409f8fef72',
   # Three lines of non-changing comments so that
   # the commit queue can handle CLs rolling ANGLE
   # and whatever else without interference from each other.
   'angle_revision': 'a4e6f07433d219fbdc802fc438a5762f53822617',
   # Three lines of non-changing comments so that
   # the commit queue can handle CLs rolling build tools
@@ skipping 966 matching lines @@
                 "--no_auth",
                 "-u",
                 "--bucket", "v8-wasm-asmjs-fuzzer",
                 "-s", "src/v8/test/fuzzer/wasm_asmjs.tar.gz.sha1",
     ],
   },
 ]
 
 # ANGLE manages DEPS that it also owns the build files for, such as dEQP.
 recursedeps = [("src/third_party/angle", "DEPS.chromium")]