chrome_paths: refactor and sanitize cache directory handling

chrome/common/chrome_paths_internal.h

Index: chrome/common/chrome_paths_internal.h
diff --git a/chrome/common/chrome_paths_internal.h b/chrome/common/chrome_paths_internal.h
index 6671e258c77fee14816bebecd8ee7eba32732848..dfe927dab44ae757e38ff3002058cc4642575775 100644
--- a/chrome/common/chrome_paths_internal.h
+++ b/chrome/common/chrome_paths_internal.h
@@ -21,6 +21,15 @@ bool GetDefaultUserDataDirectory(FilePath* result);
 // CF and Google Chrome want to share the same binaries.
 bool GetChromeFrameUserDataDirectory(FilePath* result);
 
+// Get the path to the user's cache directory.  This is normally the
+// same as the profile directory, but on Linux it can also be
+// $XDG_CACHE_HOME and on Mac it can be under ~/Library/Caches.
+// Note that the Chrome cache directories are actually subdirectories
+// of this directory, with names like "Cache" and "Media Cache".
+// This will always fill in |result| with a directory, sometimes
+// just |profile_dir|.
+void GetUserCacheDirectory(const FilePath& profile_dir, FilePath* result);
+
 // Get the path to the user's documents directory.
 bool GetUserDocumentsDirectory(FilePath* result);