Handle Windows paths for v8 debug info for builtins

If __FILE__ contained a backslash (which happened in jumbo builds on
Windows), then the generated embedded.S could contain broken strings.

This replaces backslashes with forward slashes before writing the
paths to embedded.S.

Bug: v8:8418,chromium:924454
Change-Id: I32134e9cd8acd2437f61a8f74c14583fa87a4bdf
Reviewed-on: https://chromium-review.googlesource.com/c/1430699Reviewed-by: Michael Stanton <mvstanton@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Commit-Queue: Daniel Bratell <bratell@opera.com>
Cr-Commit-Position: refs/heads/master@{#59053}

src/snapshot/embedded-file-writer.cc

@@ -4,6 +4,7 @@
 
 #include "src/snapshot/embedded-file-writer.h"
 
+#include <algorithm>
 #include <cinttypes>
 
 #include "src/objects/code-inl.h"
@@ -613,7 +614,11 @@ void PlatformDependentEmbeddedFileWriter::FilePrologue() {}
 
 void PlatformDependentEmbeddedFileWriter::DeclareExternalFilename(
     int fileid, const char* filename) {
-  fprintf(fp_, ".file %d \"%s\"\n", fileid, filename);
+  // Replace any Windows style paths (backslashes) with forward
+  // slashes.
+  std::string fixed_filename(filename);
+  std::replace(fixed_filename.begin(), fixed_filename.end(), '\\', '/');
+  fprintf(fp_, ".file %d \"%s\"\n", fileid, fixed_filename.c_str());
 }
 
 void PlatformDependentEmbeddedFileWriter::FileEpilogue() {}