Convert element name to be stored as atom instead of string.
Fix comments and variable names for atom wrapper.
use a more appropriate embedding url
Merge pull request #2864 from zmike/embedding-iscool_jpg

use a more appropriate embedding url
Merge pull request #2844 from glennw/atom-element

Convert element name to be stored as atom instead of string.
Fix issue with auto table layout when cells have multiple fragments. Fixes some layout issues related to #2554.
Add PgUp/PgDn scrolling support.
Merge pull request #2865 from glennw/wiki-layout-2

Fix issue with auto table layout when cells have multiple fragments. Fix...
Implement MessageEvent::dispatch.

This avoids duplicating the dispatching code for multiple postMessage
implementations.
Implement Worker.postMessage.
Merge pull request #2866 from glennw/pg-scroll

Add PgUp/PgDn scrolling support.
Merge pull request #2867 from Ms2ger/postMessage

Implement Worker.postMessage; r=larsberg
Don't fail on invalid JS syntax in event handlers.

Fixes #2862.
Move CSS Writing Modes properties behind a new command-line flag.
Mark as expected failure a reftest that uses 'direction'.

That property is behind the new --experimental flag.
Convert layout code to use logical directions.
Merge pull request #2869 from SimonSapin/writing-modes-rebase

Convert layout code to use logical directions (rebase)
Merge pull request #2863 from mbrubeck/2862-null-fun

Don't fail on invalid JS syntax in event handlers. r=Ms2ger
Update to reflect rust-layers changes

It is no longer necessary to flush pending buffer requests when adding
buffers nor to manually create textures for layers.
Restructure compositor layers to work with iframes

When a frame is selected via set_ids, a tree of root compositor
layers is also created, matching the tree of pipelines in the frame.
This decouples the chronological ordering dependency for parent frames
and child iframes sending CreateOrUpdateRootLayer &
CreateOrUpdateDescendentLayer messages.

Change the Compositor ready states to per pipeline and change the
compositor render states to per layer+epoch. This ensures the compositor
doesn't composite for an epoch until *every* layer in the epoch is
finished rendering.
Without tracking per layer the position_fixed_overflow ref test tends to
fail for machines under heavy load. For iframes it also fixes a bug
where the compositor didnt wait for the child pipeline to finish
rendering before compositing the window.

Gotchas:
* layout task or script task failure on exit ("task '' failed at sending
  on a closed channel"), this happens if the child iframe shares the
  same script task as the parent and can be avoided by adding the
  sandbox attribute to the iframe.

* Commented out ref test simple_iframe.html fails on os x

Other changes:
* Inline set_clipping_rect in compositor.rs, misc
* revert accidental space in glfw_windowing.rs,
* revert change in html/summit-two.html,
* dont use img in iframe_summit_ref.html,
* delete img file iframe_summit_ref.png.