Let v8 standalone builds use update.py instead of update.sh.

update.sh is gone in chromium, and using update.py will do the right
thing both before and after the deletion in chromium (previously,
update.py used to call update.sh internally on non-win).

This also has the benefit of working on Windows.

No intended behavior change.

BUG=chromium:494442
LOG=n

Review URL: https://codereview.chromium.org/1495653002

Cr-Commit-Position: refs/heads/master@{#32529}

build/standalone.gypi

@@ -415,7 +415,7 @@
         # (defines are passed via the command line, and build systems rebuild
         # things when their commandline changes). Nothing should ever read this
         # define.
-        'defines': ['CR_CLANG_REVISION=<!(<(DEPTH)/tools/clang/scripts/update.sh --print-revision)'],
+        'defines': ['CR_CLANG_REVISION=<!(python <(DEPTH)/tools/clang/scripts/update.py --print-revision)'],
         'conditions': [
           ['host_clang==1', {
             'target_conditions': [