Update the VS toolchain hash computation caching.

win_toolchain/get_toolchain_if_necessary.py

Index: win_toolchain/get_toolchain_if_necessary.py
diff --git a/win_toolchain/get_toolchain_if_necessary.py b/win_toolchain/get_toolchain_if_necessary.py
index 37a3207d8836d5840a6630a3afe9f71d0bacd5aa..aa2cf738ea1c33f5e5fb1656d992e39d0f5a0235 100755
--- a/win_toolchain/get_toolchain_if_necessary.py
+++ b/win_toolchain/get_toolchain_if_necessary.py
@@ -88,7 +88,7 @@ def GetFileList(root):
     for p in paths:
       if any(ignored_dir in p.lower() for ignored_dir in ignored_directories):
         continue
-      file_list.append(p)
+      file_list.append(p.lower())

[hans, 2016/06/08 17:07:55]

This would essentially undo https://codereview.chromium.org/2039563002/

The point of that patch was to keep the correct case in the list from
GetFileList(), because otherwise many files (e.g. DIA SDK/) won't be found on
case-sensitive filesystems.

Maybe what we want to do is make the comparison on line 130 case-insensitive.

Or, maybe better, if we do the full hash computation on line 156, call
SaveTimestampsAndHash to cache it. Currently it seems the cache is only written
after downloading a new toolchain.

   return sorted(file_list, key=lambda s: s.replace('/', '\\').lower())