Clarify documentation about importing new WPT directories.

docs/testing/web_platform_tests.md

 # Web Platform Tests
 
 The Web Platform Tests project provides a large number of conformance tests for
 different aspects of the Web Platform. Currently the tests are hosted on GitHub.
 There are two main repositories, one for the CSS Working Group (aka csswg-test),
 and one for pretty much everything else (aka web-platform-tests).
 
 There is a plan to merge csswg-test into web-platform-tests, so later, there
 will be only one repository.
 
@@ skipping 40 matching lines @@
 ### Manual import
 
 To pull the latest versions of the tests that are currently being imported, you
 can also directly invoke the
 [wpt-import](../../third_party/WebKit/Tools/Scripts/wpt-import) script.
 
 That script will pull the latest version of the tests from our mirrors of the
 upstream repositories. If any new versions of tests are found, they will be
 committed locally to your local repository. You may then upload the changes.
 
+### Enabling import for a new directory
+
 If you wish to add more tests (by un-skipping some of the directories currently
 skipped in `W3CImportExpectations`), you can modify that file locally and commit
-it, and on the next auto-import, the new tests should be imported. If you want
-to import immediately, you can also run `wpt-import --allow-local-commits`.
+it, and on the next auto-import, the new tests should be imported.
+
+If you want to import immediately (in order to try the tests out locally, etc)
+you can also run `wpt-import --allow-local-commits`, but this is not required.
 
 ## Contributing tests back to the Web Platform Tests project.
 
 If you need to make changes to Web Platform Tests, just commit your changes
 directly to
 [LayoutTests/external/wpt](../../third_party/WebKit/LayoutTests/external/wpt)
 and the changes will be automatically upstreamed within 24 hours.
 
 Note: if you're adding a new test in `external/wpt`, you'll need to re-generate
 MANIFEST.json manually until [CL 2644783003](https://crrev.com/2644783003) is
 landed. The command to do so is:
 
 ```bash
 Tools/Scripts/webkitpy/thirdparty/wpt/wpt/manifest --work \
     --tests-root=LayoutTests/external/wpt
 ```
 
+### What kinds of changes can be upstreamed?
+
+In general, changes involving adding, removing or modifying tests can all be
+upstreamed. From a Chromium commit, any changes outside of
+[external/wpt](../../third_party/WebKit/LayoutTests/external/wpt) will not be
+upstreamed, and any changes `*-expected.txt`, `OWNERS`, and `MANIFEST.json`,
+will also not be upstreamed.
+
+An entirely new top-level directory could be added in a Chromium commit,
+although in general you won't need to do this. If this is done, then an entry
+should be added to `W3CImportExpectations`.

[foolip, 2017/02/10 23:43:33]

I think we should recommend doing this upstream since that's the only way to add
a correct OWNERS file. It is of course OK to do that after adding the initial
tests.

[qyearsley, 2017/02/10 23:56:24]

Makes sense, now changed this recommendation.

+
+### Web Platform Tests contributing guidelines
+
+The Web Platform Tests has [guidelines for
+contributors](https://github.com/w3c/web-platform-tests/blob/master/CONTRIBUTING.md).

[foolip, 2017/02/10 23:43:33]

This documentation is just legal stuff, I think linking to it isn't very
helpful. Perhaps once gsnedders has refactored some documentation we can link to
that.

I was also tweaking this documentation, this is what I had written:

Most tests are written using testharness.js, see
[Writing Layout Tests](./writing_layout_tests.md) and
[Layout Tests Tips](./layout_tests_tips.md) for general guidelines.

[qyearsley, 2017/02/10 23:56:24]

For this CL, I'll just remove this section; we could expand it later.

+Remember that in general, tests correspond to specs, and tests in Web Platform

[Rick Byers, 2017/02/10 18:15:52]

I guess I'd suggest keeping everything above, and just using the (slightly more
detailed) paragraph I wrote for this in place of these 3 lines.

[qyearsley, 2017/02/10 23:56:24]

Removing these 3 lines

+Tests are intended to be cross-browser testing for conformance to
+specifications.
+
 ### Will the exported commits be linked to my GitHub profile?
 
 The email you commit with in Chromium will be the author of the commit on
 GitHub. You can [add it as a secondary address on your GitHub
 account](https://help.github.com/articles/adding-an-email-address-to-your-github-account/)
 to link your exported commits to your GitHub profile.
 
 ### What if there are conflicts?
 
 This cannot be avoided entirely as the two repositories are independent, but
 should be rare with frequent imports and exports. When it does happen, manual
 intervention will be needed and in non-trivial cases you may be asked to help
 resolve the conflict.
 
 ### Direct pull requests
 
 It's still possible to make direct pull requests to web-platform-tests. The
 processes for getting new tests committed the W3C repos are at
 http://testthewebforward.org/docs/. Some specifics are at
 http://testthewebforward.org/docs/github-101.html.