Port selector matching to use atoms.

Removed the Lowercase new type. This does mean that this
code path is slightly slower for now. This is because
find_equiv() doesn't work with atoms in hash tables, due to
the hash value being different than that of a string. However
it also means the removal of some unsafe code, and a better
long term solution will be to precache the lower case versions
as atoms.
Update wpt expected results.

Due to a mozlog bug, our automation didn't catch those changes.
Update README.md
Merge pull request #3064 from Ms2ger/fix-wpt

Update wpt expected results; r=jdm
Throw a catchable exception from ThrowingConstructor.
Merge pull request #3068 from Ms2ger/throwing-ctor

Throw a catchable exception from ThrowingConstructor; r=Manishearth
Merge pull request #3067 from Manishearth/fedora-devel

Update README.md
Refactor how LayoutContext structure works (reduce TLS lookups + simplify fns used by seq/parallel code paths).

- LayoutContext is renamed to SharedLayoutContext.
- SharedLayoutContext is immutable.
- LayoutContext is a wrapper around SharedLayoutContext + access to local caches (font, style etc).
- Creating a LayoutContext does a single local_data lookup to fetch the cache information.
- Android shares same implementation of context.rs as other platforms.
- LayoutContext can be used from both green thread (parallel layout) and native thread (sequential layout).
- Removes the need for other types (such as FontContext, StyleSharingCandidateCache etc) to be passed around.
Update rust-layers with android glsl fix.
Merge pull request #3070 from glennw/android-rust-layers

Update rust-layers with android glsl fix.
Merge pull request #3069 from glennw/refactor_layout_context

Refactor how LayoutContext structure works (reduce TLS lookups + simplify fns used by seq/parallel code paths).
Give workers their own ScriptChan and use it for postMessage.

This ensures that XHR callbacks for XHR objects in workers are called on the
worker thread rather than the main thread.
Merge commit 'refs/pull/3029/head' of https://github.com/servo/servo

Conflicts:
	src/components/style/selector_matching.rs
Improve the documentation for ScriptMsg.
Merge pull request #3071 from Ms2ger/workers-threads

Give workers their own ScriptChan and use it for postMessage; r=Manishearth
Change line-height to be read from the font itself, rather than
a hard-coded estimate.
Merge pull request #3074 from glennw/font-line-height

Change line-height to be read from the font itself, rather than a hard-coded estimate.
Add ahem test font for ref tests to use. Add ref test for line-height using ahem as web font.
Make libscript compilation more verbose on Travis.
Merge pull request #3072 from Ms2ger/libscript-log-travis

Make libscript compilation more verbose on Travis; r=Manishearth+SimonSapin