Revert of Pick a fixed Clang revision for Windows now that we have an LKGR...

Revert of Pick a fixed Clang revision for Windows now that we have an LKGR ASan builder (https://codereview.chromium.org/320383004/)

Reason for revert:
This was landed despite an explicit "not lgtm"

Original issue's description:
> Pick a fixed Clang revision for Windows now that we have an LKGR ASan builder
> This way, the LKGR builder and the developers who want to try ASan (or just Clang) on Windows won't get a broken Clang revision.
> 
> The FYI bots got LLVM_WIN_REVISION=HEAD in r276301 so won't be affected.
> 
> BUG=82385,345874
> NOTRY=true
> 
> Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=276366

TBR=hans@chromium.org,rnk@chromium.org,timurrrr@chromium.org
NOTREECHECKS=true
NOTRY=true
BUG=82385,345874

Review URL: https://codereview.chromium.org/342533002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@277837 0039d316-1c4b-4281-b951-d872f2087c98

tools/clang/scripts/update.py

@@ -15,9 +15,8 @@ import sys
 # Do NOT CHANGE this if you don't know what you're doing -- see
 # https://code.google.com/p/chromium/wiki/UpdatingClang
 # Reverting problematic clang rolls is safe, though.
-# TODO(hans): update the wiki to cover Windows when Clang is stable enough.
 # Note: this revision is only used for Windows. Other platforms use update.sh.
-LLVM_WIN_REVISION = os.environ.get('LLVM_WIN_REVISION', '210586')
+LLVM_WIN_REVISION = 'HEAD'
 
 # Path constants. (All of these should be absolute paths.)
 THIS_DIR = os.path.abspath(os.path.dirname(__file__))
@@ -30,8 +29,9 @@ CLANG_DIR = os.path.join(LLVM_DIR, 'tools', 'clang')
 COMPILER_RT_DIR = os.path.join(LLVM_DIR, 'projects', 'compiler-rt')
 STAMP_FILE = os.path.join(LLVM_BUILD_DIR, 'cr_build_revision')
 
-LLVM_REPO_URL = os.environ.get('LLVM_REPO_URL',
-                               'https://llvm.org/svn/llvm-project')
+LLVM_REPO_URL='https://llvm.org/svn/llvm-project'
+if 'LLVM_REPO_URL' in os.environ:
+  LLVM_REPO_URL = os.environ['LLVM_REPO_URL']
 
 
 def ReadStampFile():