GTK: Refactor some things so GTK doesn't touch the on disk image cache.

chrome/browser/browser_theme_provider.h

Index: chrome/browser/browser_theme_provider.h
diff --git a/chrome/browser/browser_theme_provider.h b/chrome/browser/browser_theme_provider.h
index d6c20b65fd79dd9ed3f00473678a54e11fbc892c..0d3524bbe08b6078224e60ccbe57f33bf62e5255 100644
--- a/chrome/browser/browser_theme_provider.h
+++ b/chrome/browser/browser_theme_provider.h
@@ -261,6 +261,11 @@ class BrowserThemeProvider : public NonThreadSafe,
   // from ClearCaches().
   virtual void FreePlatformCaches();
 
+  // The implementation of GenerateTabBackgroundBitmap(). That function also
+  // must be locked and touches caches; this function only deals with image
+  // generation.
+  SkBitmap* GenerateTabBackgroundBitmapImpl(int id);
+
   Profile* profile() { return profile_; }
 
   // Subclasses may need us to not use the on-disk image cache. The GTK