Force python 2.7 on TravisCI.
From TravisCI:
```
This job is running using the default Python version, which will be updated to 3.6 on April 16th, 2019. You can explicitly stay on the previous version by specifying python: 2.7 in your .travis.yml.
```
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Replace panic with warn in DocumentLoader.finish_load.
Fix panic on broken script URL with an onerror handler that rewrites the document.
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Refactoring RequestInit to use a Builder Pattern
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If RequestInit::new accepts all of the mandatory arguments and then the builder pattern is used for customizable options, the resulting code might be easier to match against specification text like
---
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Review WPT sync PRs automatically
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- [x] These changes fix#23113
- [x] These changes do not require tests because it's about infrastructure
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Add default paths for WPT tests
Set the following default configs for WPT tests.
```--ca-cert-path=tests\wpt\web-platform-tests\tools\certs\cacert.pem
--host-key-path=tests\wpt\web-platform-tests\tools\certs\web-platform.test.key
--host-cert-path=tests\wpt\web-platform-tests\tools\certs\web-platform.test.pem
--ssl-type=pregenerated
```
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- [X] These changes fix#23133
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Add a note to indicate PKG_CONFIG_PATH required
I just got the error messages like #23015 again and then I think the root cause should be about
`PKG_CONFIG_PATH`. After following https://github.com/servo/servo/issues/23015#issuecomment-475050175 said, I can build Servo successfully.
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Promise constructor
In this PR, measures are taken that prevent the ```Promise::new``` constructor from being used outside a compartment.
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Add XRFrame.getPose()
I think I've figured out the model of poses, waiting on Nell for confirmation.
Basically, `getViewerPose(p)` is equivalent to `getPose(source=viewerSpace, relative_to=p)`
The eye-level space, for example, is stationary and stuck to the origin. The position-disabled and identity spaces somewhat counterintuitively follow you around (and appear to be stationary from `getViewerPose()` but not `getPose()`.
The incorrect mental model kinda "works" when looking at only `getViewerPose()`, but we need to figure it out for `getPose()`.
Todo (may add to this PR, but probably not)
- implement `XRSession.viewerSpace`
- implement position-disabled
- implement floor-level (hard to test without a 6dof device)
r? @asajeffrey
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Remove compound microtasks
We handled compound microtasks as microtasks so, basically, we only need
to remove the naming of `compound`.
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- [x] These changes fix#23140
- [x] These changes do not require tests because the updated spec is more about editorial.
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