Fix and enable mojo:media_pipeline_integration_apptests.

Currently the mojo implementation doesn't fully support EME nor
clockless playback, so those tests are disabled for mojo.

BUG=501417
TEST=new tests pass

Review URL: https://codereview.chromium.org/1591103002

Cr-Commit-Position: refs/heads/master@{#371840}

diff --git a/BUILD.gn b/BUILD.gn
index fac0464..d644a5d 100644
--- a/BUILD.gn
+++ b/BUILD.gn

@@ -614,6 +614,7 @@
     deps += [
       "//mandoline:tests",
       "//mash/wm:tests",
+      "//media/mojo/services:tests",
       "//ui/views/mus:tests",
     ]
   }