Sort project headers in a case-sensitive way since PRESUBMIT enforces that

tools/sort-headers.py

Index: tools/sort-headers.py
diff --git a/tools/sort-headers.py b/tools/sort-headers.py
index b1575b3822b871ce831428f5dd842399aa1b62ba..ac20ca1cfb11e14fa71f6d971b9622e62fa38c04 100755
--- a/tools/sort-headers.py
+++ b/tools/sort-headers.py
@@ -37,7 +37,6 @@ def IncludeCompareKey(line):
   """Sorting comparator key used for comparing two #include lines.
   Returns the filename without the #include/#import prefix.
   """
-  line = line.lower()
   for prefix in ('#include ', '#import '):
     if line.startswith(prefix):
       line = line[len(prefix):]
@@ -56,9 +55,9 @@ def IncludeCompareKey(line):
   # C++ system headers should come after C system headers.
   if line.startswith('<'):
     if line.find('.h>') != -1:
-      return '2' + line
+      return '2' + line.lower()
     else:
-      return '3' + line
+      return '3' + line.lower()
 
   return '4' + line