Update from https://crrev.com/328418

tools/clang/plugins/ChromeClassTester.cpp

Index: tools/clang/plugins/ChromeClassTester.cpp
diff --git a/tools/clang/plugins/ChromeClassTester.cpp b/tools/clang/plugins/ChromeClassTester.cpp
index 931da02f3ff54e352a1cbec4337433abb38e449f..6a4233a659a5e3fc4f8ad57da936be93097e85c7 100644
--- a/tools/clang/plugins/ChromeClassTester.cpp
+++ b/tools/clang/plugins/ChromeClassTester.cpp
@@ -18,6 +18,7 @@
 #endif
 
 using namespace clang;
+using chrome_checker::Options;
 
 namespace {
 
@@ -30,8 +31,10 @@ bool ends_with(const std::string& one, const std::string& two) {
 
 }  // namespace
 
-ChromeClassTester::ChromeClassTester(CompilerInstance& instance)
-    : instance_(instance),
+ChromeClassTester::ChromeClassTester(CompilerInstance& instance,
+                                     const Options& options)
+    : options_(options),
+      instance_(instance),
       diagnostic_(instance.getDiagnostics()) {
   BuildBannedLists();
 }
@@ -95,16 +98,13 @@ void ChromeClassTester::emitWarning(SourceLocation loc,
   std::string err;
   err = "[chromium-style] ";
   err += raw_error;
-  // TODO(dcheng): Re-enable -Werror for these diagnostics on Windows once all
-  // the pre-existing warnings are cleaned up. https://crbug.com/467287
-  DiagnosticIDs::Level level =
-#if !defined(LLVM_ON_WIN32)
-      diagnostic().getWarningsAsErrors() ?
-      DiagnosticIDs::Error :
-#endif
-      DiagnosticIDs::Warning;
+
+  DiagnosticIDs::Level level = getErrorLevel() == DiagnosticsEngine::Error
+      ? DiagnosticIDs::Error : DiagnosticIDs::Warning;
+
   unsigned id = diagnostic().getDiagnosticIDs()->getCustomDiagID(level, err);
   DiagnosticBuilder builder = diagnostic().Report(full, id);
+
 }
 
 bool ChromeClassTester::InBannedDirectory(SourceLocation loc) {
@@ -130,9 +130,7 @@ bool ChromeClassTester::InBannedDirectory(SourceLocation loc) {
   }
 
 #if defined(LLVM_ON_UNIX)
-  // We need to munge the paths so that they are relative to the repository
-  // srcroot. We first resolve the symlinktastic relative path and then
-  // remove our known srcroot from it if needed.
+  // Resolve the symlinktastic relative path and make it absolute.
   char resolvedPath[MAXPATHLEN];
   if (realpath(filename.c_str(), resolvedPath)) {
     filename = resolvedPath;
@@ -141,6 +139,15 @@ bool ChromeClassTester::InBannedDirectory(SourceLocation loc) {
 
 #if defined(LLVM_ON_WIN32)
   std::replace(filename.begin(), filename.end(), '\\', '/');
+
+  // On Posix, realpath() has made the path absolute.  On Windows, this isn't
+  // necessarily true, so prepend a '/' to the path to make sure the
+  // banned_directories_ loop below works correctly.
+  // This turns e.g. "gen/dir/file.cc" to "/gen/dir/file.cc" which lets the
+  // "/gen/" banned_dir work.
+  // This seems simpler than converting to utf16, calling GetFullPathNameW(),
+  // and converting back to utf8.
+  filename.insert(filename.begin(), '/');
 #endif
 
   for (const std::string& banned_dir : banned_directories_) {
@@ -304,3 +311,11 @@ bool ChromeClassTester::GetFilename(SourceLocation loc,
   *filename = ploc.getFilename();
   return true;
 }
+
+DiagnosticsEngine::Level ChromeClassTester::getErrorLevel() {
+  if (options_.warn_only)
+    return DiagnosticsEngine::Warning;
+
+  return diagnostic().getWarningsAsErrors() ? DiagnosticsEngine::Error
+                                            : DiagnosticsEngine::Warning;
+}