Change update-w3c-test-expectations to only rebaseline existing tests.

Change update-w3c-test-expectations to only rebaseline existing tests.

In this CL:
 - Extract a method get_modified_existing_tests which outputs warnings
   for tests that don't exist.
 - In get_modified_existing_tests, when invoking git diff, add --diff-filter
   so that tests that don't exist shouldn't be gotten in the first place.
 - Update tests and expand test coverage.

BUG=649691
Committed: https://crrev.com/c4e50671c1d6124c8cc4b315f9308a365da79df4
Cr-Commit-Position: refs/heads/master@{#423227}

[qyearsley, 2016-10-03 17:30:47]

qyearsley@chromium.org changed reviewers:
+ jeffcarp@chromium.org, jsbell@chromium.org

[qyearsley, 2016-10-04 23:29:13]

The CQ bit was checked by qyearsley@chromium.org to run a CQ dry run

[commit-bot: I haz the power, 2016-10-04 23:29:45]

Dry run: CQ is trying da patch. Follow status at
 
https://chromium-cq-status.appspot.com/v2/patch-status/codereview.chromium.or...

[jsbell, 2016-10-04 23:43:34]

lgtm with nits

https://codereview.chromium.org/2389873002/diff/1/third_party/WebKit/Tools/Sc...
File
third_party/WebKit/Tools/Scripts/webkitpy/w3c/update_w3c_test_expectations.py
(right):

https://codereview.chromium.org/2389873002/diff/1/third_party/WebKit/Tools/Sc...
third_party/WebKit/Tools/Scripts/webkitpy/w3c/update_w3c_test_expectations.py:351:
A pair: A set of tests to be rebaselined, and an modified updated
Grammar: "an modified updated" doesn't make sense

https://codereview.chromium.org/2389873002/diff/1/third_party/WebKit/Tools/Sc...
File
third_party/WebKit/Tools/Scripts/webkitpy/w3c/update_w3c_test_expectations_unittest.py
(right):

https://codereview.chromium.org/2389873002/diff/1/third_party/WebKit/Tools/Sc...
third_party/WebKit/Tools/Scripts/webkitpy/w3c/update_w3c_test_expectations_unittest.py:6:
import copy
sort imports alphabetically?

[commit-bot: I haz the power, 2016-10-05 00:40:14]

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

[commit-bot: I haz the power, 2016-10-05 00:40:14]

Dry run: This issue passed the CQ dry run.

[qyearsley, 2016-10-05 17:20:36]

https://codereview.chromium.org/2389873002/diff/1/third_party/WebKit/Tools/Sc...
File
third_party/WebKit/Tools/Scripts/webkitpy/w3c/update_w3c_test_expectations.py
(right):

https://codereview.chromium.org/2389873002/diff/1/third_party/WebKit/Tools/Sc...
third_party/WebKit/Tools/Scripts/webkitpy/w3c/update_w3c_test_expectations.py:351:
A pair: A set of tests to be rebaselined, and an modified updated
On 2016/10/04 at 23:43:33, jsbell wrote:
> Grammar: "an modified updated" doesn't make sense

Gah, thanks for catching :-) The intended change here was to say that instead of
updating the original dictionary and returning it, this now returns a separate
copy based on the original; the reason for this change is because that is what I
expected the behavior should be like originally.

https://codereview.chromium.org/2389873002/diff/1/third_party/WebKit/Tools/Sc...
File
third_party/WebKit/Tools/Scripts/webkitpy/w3c/update_w3c_test_expectations_unittest.py
(right):

https://codereview.chromium.org/2389873002/diff/1/third_party/WebKit/Tools/Sc...
third_party/WebKit/Tools/Scripts/webkitpy/w3c/update_w3c_test_expectations_unittest.py:6:
import copy
On 2016/10/04 at 23:43:33, jsbell wrote:
> sort imports alphabetically?

Done

[qyearsley, 2016-10-05 17:20:43]

The CQ bit was checked by qyearsley@chromium.org

[qyearsley, 2016-10-05 17:20:44]

The patchset sent to the CQ was uploaded after l-g-t-m from jsbell@chromium.org
Link to the patchset: https://codereview.chromium.org/2389873002/#ps20001
(title: "Sort imports, fix grammar issue")

[commit-bot: I haz the power, 2016-10-05 17:21:11]

CQ is trying da patch. Follow status at
 
https://chromium-cq-status.appspot.com/v2/patch-status/codereview.chromium.or...

[commit-bot: I haz the power, 2016-10-05 18:26:50]

Committed patchset #2 (id:20001)

[commit-bot: I haz the power, 2016-10-05 18:30:59]

Description was changed from

==========
Change update-w3c-test-expectations to only rebaseline existing tests.

In this CL:
 - Extract a method get_modified_existing_tests which outputs warnings
   for tests that don't exist.
 - In get_modified_existing_tests, when invoking git diff, add --diff-filter
   so that tests that don't exist shouldn't be gotten in the first place.
 - Update tests and expand test coverage.

BUG=649691
==========

to

==========
Change update-w3c-test-expectations to only rebaseline existing tests.

In this CL:
 - Extract a method get_modified_existing_tests which outputs warnings
   for tests that don't exist.
 - In get_modified_existing_tests, when invoking git diff, add --diff-filter
   so that tests that don't exist shouldn't be gotten in the first place.
 - Update tests and expand test coverage.

BUG=649691

Committed: https://crrev.com/c4e50671c1d6124c8cc4b315f9308a365da79df4
Cr-Commit-Position: refs/heads/master@{#423227}
==========

[commit-bot: I haz the power, 2016-10-05 18:31:00]

Patchset 2 (id:??) landed as
https://crrev.com/c4e50671c1d6124c8cc4b315f9308a365da79df4
Cr-Commit-Position: refs/heads/master@{#423227}