Add declaration for DictionaryValue in devtools_http_handler.h

Without above change it breaks the build for Crosswalk which is dependent on
content and components like devtools_http_handler. This is somehow not evident
on Chrome or Content Shell builds which might be including base/values.h through
some other includes.

R=pfeldman@chromium.org
BUG=

Review URL: https://codereview.chromium.org/1137243005

Cr-Commit-Position: refs/heads/master@{#330581}

components/devtools_http_handler/devtools_http_handler.h

@@ -17,6 +17,7 @@
 class GURL;
 
 namespace base {
+class DictionaryValue;
 class Thread;
 class Value;
 }