PrecacheFetcher::CancelPrecaching should run in the main UI thread

components/precache/core/precache_fetcher.cc

Index: components/precache/core/precache_fetcher.cc
diff --git a/components/precache/core/precache_fetcher.cc b/components/precache/core/precache_fetcher.cc
index dc0d3a272327a9ce1002ba33b377b2abb2f7e058..dd329985ed70370f5516a9b38b18f25e83515d95 100644
--- a/components/precache/core/precache_fetcher.cc
+++ b/components/precache/core/precache_fetcher.cc
@@ -359,6 +359,9 @@ PrecacheFetcher::~PrecacheFetcher() {
 }
 
 std::unique_ptr<PrecacheUnfinishedWork> PrecacheFetcher::CancelPrecaching() {
+  if (!unfinished_work_)
+    return nullptr;

[Raj, 2016/07/08 17:57:35]

CancelPrecaching() could get called multiple times, though I do not see an
apparent codepath for multiple calls.
But it should be handled gracefully.

[sclittle, 2016/07/11 18:16:51]

Could you add this comment to the code too?

+
   unfinished_work_->clear_manifest();
   unfinished_work_->clear_resource();
   for (const auto& manifest : manifest_urls_to_fetch_)