Sync WPT with upstream (15-03-2019)

[servo-wpt-sync]

Automated downstream sync of changes from upstream as of 15-03-2019.
[no-wpt-sync]

---

This change is 

[jdm]

@bors-servo r+

[bors-servo]

📌 Commit 86523cd has been approved by jdm

[bors-servo]

⌛ Testing commit 86523cd with merge 6bd69bc...

[bors-servo]

💔 Test failed - linux-rel-css

[CYBAI]

Failure in Travis

36.71s$ ./mach test-tidy --no-progress --all
 0:07.10 INFO Diffing old and new manifests /home/travis/build/servo/servo/tests/wpt/mozilla/meta/MANIFEST.json
Error running mach:
    ['test-tidy', '--no-progress', '--all']
The error occurred in code that was called by the mach command. This is either
a bug in the called code itself or in the way that mach is calling it.
You should consider filing a bug for this issue.
If filing a bug, please include the full output of mach, including this error
message.
The details of the failure are as follows:
AttributeError: 'SupportFile' object has no attribute 'meta_key'
  File "/home/travis/build/servo/servo/python/servo/testing_commands.py", line 330, in test_tidy
    manifest_dirty = run_update(self.context.topdir, check_clean=True)
  File "/home/travis/build/servo/servo/python/servo/testing_commands.py", line 90, in run_update
    return manifestupdate.update(logger, wpt_dir, check_clean, rebuild)
  File "/home/travis/build/servo/servo/tests/wpt/manifestupdate.py", line 54, in update
    return _check_clean(logger, test_paths)
  File "/home/travis/build/servo/servo/tests/wpt/manifestupdate.py", line 86, in _check_clean
    if not diff_manifests(logger, manifest_path, old_manifest, new_manifest):
  File "/home/travis/build/servo/servo/tests/wpt/manifestupdate.py", line 113, in diff_manifests
    for item in test.meta_key())
The command "./mach test-tidy --no-progress --all" exited with 1.

and the linux-rel-css failed with

  ▶ FAIL [expected PASS] /css/CSS2/text/white-space-003.xht
  └   → /css/CSS2/text/white-space-003.xht 176a7d7217da6442e19d9c32b5940424189a8bd0
/css/CSS2/text/white-space-001-ref.xht be47dca27b1ba0f26939434b91c1ba8ae840201b

[CYBAI]

Close this one because @jdm is working on fixing WPT sync at #23051