Truncate the download file name if it exceeds the filesystem limit.

base/file_util.h

Index: base/file_util.h
diff --git a/base/file_util.h b/base/file_util.h
index 254f32607b6b0064b20aed4092dcbe38cfe84f8d..6a8fd052bf753ac7f65b63e5661cc5dbb8b99b0f 100644
--- a/base/file_util.h
+++ b/base/file_util.h
@@ -422,6 +422,10 @@ BASE_EXPORT bool VerifyPathControlledByUser(const base::FilePath& base,
 BASE_EXPORT bool VerifyPathControlledByAdmin(const base::FilePath& path);
 #endif  // defined(OS_MACOSX) && !defined(OS_IOS)
 
+// Returns the maximum length of path component on the volume containing
+// the directory |path|, in the number of FilePath::CharType, or -1 on failure.
+BASE_EXPORT int GetMaximumPathComponentLength(const base::FilePath& path);
+
 // A class to handle auto-closing of FILE*'s.
 class ScopedFILEClose {
  public: