Disable ProxyScriptFetcherImplTest.HttpMimeType.

[Chromecast] Disable ProxyScriptFetcherImplTest.HttpMimeType.

ProxyScriptFetcherImplTest.HttpMimeType is flaky. Since proxy
script support isn't a huge priority, disable test for now.

BUG=internal b/19848784
R=gunsch@chromium.org

Committed: https://crrev.com/c4c495eb094ea92a593580d361688e8aa3bae79b
Cr-Commit-Position: refs/heads/master@{#322109}

[gunsch, 2015-03-24 22:14:02]

lgtm

[derekjchow1, 2015-03-24 22:15:46]

The CQ bit was checked by derekjchow@chromium.org

[derekjchow1, 2015-03-24 22:15:57]

The CQ bit was unchecked by derekjchow@chromium.org

[derekjchow1, 2015-03-24 22:16:09]

The CQ bit was checked by derekjchow@chromium.org

[commit-bot: I haz the power, 2015-03-24 22:17:09]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1036683002/1

[commit-bot: I haz the power, 2015-03-25 01:17:46]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-03-25 01:18:59]

Patchset 1 (id:??) landed as
https://crrev.com/c4c495eb094ea92a593580d361688e8aa3bae79b
Cr-Commit-Position: refs/heads/master@{#322109}