Update comments about Node::layout_data
Merge pull request #2919 from mbrubeck/layout-comments

Update comments about Node::layout_data
Simplify set_clipping_rect and inline it into its only caller
Merge pull request #2928 from zwarich/set-clipping-cleanup

Simplify set_clipping_rect and inline it into its only caller
Add pointers to the relevant issue for some bugs.
Add documentation for the DOM.
Merge pull request #2931 from Ms2ger/dom-docs

Add documentation for the DOM; r=jdm
Assert that we're handed a DOM global in initialize_global.
Merge pull request #2932 from Ms2ger/fixme-347

Add pointers to the relevant issue for some bugs; r=jdm
Track rerender msgs

If the compositor outputs to a file:
* Track rerender msgs sent from the compositor to the render tasks.
* Before outputting, wait until all rerender msgs are processed by the
  render tasks.

Fixes issue https://github.com/servo/servo/issues/2871.
Merge pull request #2930 from bjwbell/track-rerender-msgs

Track rerender msgs
Implement Location.hash.
Merge pull request #2935 from Ms2ger/location-hash

Implement Location.hash.
Merge pull request #2933 from Ms2ger/initialize_global

Assert that we're handed a DOM global in initialize_global.
Pass a non-nullable string to NamedGetter.

There is no actual reason to use a nullable string here; all callers have a
string they want to pass.

The issue dates back to the time that DOMString was inherently nullable
(before #1215); this API was not converted back to the non-nullable DOMString
type after that landed.
Remove an unused argument to needCx and use it in CGCallGenerator.
Introduce a PerformanceTimingHelpers trait for the NavigationStartPrecise method.
Remove references to nullable any from testbinding.

'any' is inherently nullable, so nullable any doesn't exist.
Comment out references to floating-point arguments with default values.

Our parser does not support floating-point constants, so we can't use default
values for floating-point types.
Add methods that return floating-point types to TestBinding.webidl.

They were already implemented in testbinding.rs, but were forgotten in the IDL
file.