Add sequencing comment to PpdCacheImpl.

chromeos/printing/ppd_cache.cc

Index: chromeos/printing/ppd_cache.cc
diff --git a/chromeos/printing/ppd_cache.cc b/chromeos/printing/ppd_cache.cc
index 918b0e84cf9426a14b1565dab0dc7520354cf35a..653c56bff1cb92747e7369db1a2249562518e268 100644
--- a/chromeos/printing/ppd_cache.cc
+++ b/chromeos/printing/ppd_cache.cc
@@ -116,6 +116,11 @@ void StoreImpl(const base::FilePath& cache_dir,
   }
 }
 
+// Implementation of the PpdCache that uses two separate task runners for Store
+// and Fetch since the two operations have different priorities. Note that the
+// two operations are not sequenced so there should be no expectation that a
+// call to Find will return a file that was previously Stored until the Store
+// callback is run.
 class PpdCacheImpl : public PpdCache {
  public:
   explicit PpdCacheImpl(const base::FilePath& cache_base_dir)