Add declaration for DictionaryValue in devtools_http_handler.h

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=

Committed: https://crrev.com/eb8ea8d2984b4177cb7bc055efd0fd10859317fe
Cr-Commit-Position: refs/heads/master@{#330581}

[mrunal, 2015-05-19 01:26:20]

mrunal.kapade@intel.com changed reviewers:
+ pfeldman@chromium.org

[pfeldman, 2015-05-19 03:56:55]

lgtm

[mrunal, 2015-05-19 17:55:44]

The CQ bit was checked by mrunal.kapade@intel.com

[commit-bot: I haz the power, 2015-05-19 17:56:32]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1137243005/1

[commit-bot: I haz the power, 2015-05-19 19:25:05]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-05-19 19:25:49]

Patchset 1 (id:??) landed as
https://crrev.com/eb8ea8d2984b4177cb7bc055efd0fd10859317fe
Cr-Commit-Position: refs/heads/master@{#330581}