Remove references to WebMidiClientMock from content/.

Remove references to WebMidiClientMock from content/.

It was only used for permissions mocking and there is a new TestRunner
API that handles this.

This CL is part of a suite of four patches:
Part 1: https://codereview.chromium.org/1100843003
Part 2: <this>
Part 3: https://codereview.chromium.org/1097933006
Part 4: https://codereview.chromium.org/1095233005

BUG=475141
Committed: https://crrev.com/835cbf31ec2e797407306b672cb33dd2d60e5cdd
Cr-Commit-Position: refs/heads/master@{#326491}

[mlamouri (slow - plz ping), 2015-04-22 14:41:56]

mlamouri@chromium.org changed reviewers:
+ mkwst@chromium.org

[mlamouri (slow - plz ping), 2015-04-22 14:41:57]

Mike, I've heard you like line removals so I have a few for you here.

[Mike West, 2015-04-22 14:50:15]

LGTM! More deletions!

[mlamouri (slow - plz ping), 2015-04-23 10:03:30]

The CQ bit was checked by mlamouri@chromium.org

[mlamouri (slow - plz ping), 2015-04-23 10:03:31]

The patchset sent to the CQ was uploaded after l-g-t-m from mkwst@chromium.org
Link to the patchset: https://codereview.chromium.org/1100673004/#ps20001
(title: "update")

[commit-bot: I haz the power, 2015-04-23 10:03:42]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1100673004/20001

[commit-bot: I haz the power, 2015-04-23 10:46:13]

Committed patchset #2 (id:20001)

[commit-bot: I haz the power, 2015-04-23 10:47:22]

Patchset 2 (id:??) landed as
https://crrev.com/835cbf31ec2e797407306b672cb33dd2d60e5cdd
Cr-Commit-Position: refs/heads/master@{#326491}