Avoid using DictionaryPrefUpdate for read-only pref ops.

ExternalProtocolHandler::GetBlockState queries for a value in a dict
pref. Previously, it was using DictionaryPrefUpdate to get the
dict. This causes the pref file to be written out in its dtor. This CL
switches to ordinary read-only ops to avoid spurious pref file writes.

BUG=none
R=benwells@chromium.org

Change-Id: I9a42441cdee70e7bd9b0f67afefb67d411bef2dd
Reviewed-on: https://chromium-review.googlesource.com/c/1298715Reviewed-by: Ben Wells <benwells@chromium.org>
Commit-Queue: Greg Thompson <grt@chromium.org>
Cr-Commit-Position: refs/heads/master@{#603041}

chrome/browser/external_protocol/external_protocol_handler.cc

@@ -188,8 +188,8 @@ ExternalProtocolHandler::BlockState ExternalProtocolHandler::GetBlockState(
 
   PrefService* profile_prefs = profile->GetPrefs();
   if (profile_prefs) {  // May be NULL during testing.
-    DictionaryPrefUpdate update_excluded_schemas_profile(
-        profile_prefs, prefs::kExcludedSchemes);
+    const base::DictionaryValue* update_excluded_schemas_profile =
+        profile_prefs->GetDictionary(prefs::kExcludedSchemes);
     bool should_block;
     // Ignore stored block decisions. These are now not possible through the UI,
     // and previous block decisions should be ignored to allow users to recover