add missing content_test_mojo_bindings gyp dep in content_tests.gypi

add missing content_test_mojo_bindings gyp dep in content_tests.gypi

This should avoid possible build flakiness.

Found using this proposed ninja patch:
https://github.com/martine/ninja/pull/1031

BUG=536641
Committed: https://crrev.com/6a65ae6ee1d7ed4ac5c4311ed2901c3ba9a4e598
Cr-Commit-Position: refs/heads/master@{#351063}

[Mostyn Bramley-Moore, 2015-09-28 11:23:17]

The CQ bit was checked by mostynb@opera.com to run a CQ dry run

[commit-bot: I haz the power, 2015-09-28 11:23:31]

Dry run: CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1371073002/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1371073002/1

[Mostyn Bramley-Moore, 2015-09-28 11:25:40]

mostynb@opera.com changed reviewers:
+ jochen@chromium.org

[Mostyn Bramley-Moore, 2015-09-28 11:25:41]

@Jochen: please take a look at this build flakiness fixup.

[jochen (gone - plz use gerrit), 2015-09-28 11:39:37]

lgtm

[commit-bot: I haz the power, 2015-09-28 13:35:34]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2015-09-28 13:35:35]

Dry run: This issue passed the CQ dry run.

[Mostyn Bramley-Moore, 2015-09-28 13:44:42]

The CQ bit was checked by mostynb@opera.com

[commit-bot: I haz the power, 2015-09-28 13:45:08]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1371073002/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1371073002/1

[commit-bot: I haz the power, 2015-09-28 13:50:10]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-09-28 13:50:59]

Patchset 1 (id:??) landed as
https://crrev.com/6a65ae6ee1d7ed4ac5c4311ed2901c3ba9a4e598
Cr-Commit-Position: refs/heads/master@{#351063}