Remove shadow trees and custom elements from sky/engine/.

Try building Release with -j 2

Remove all code relating to shadow trees, insertion points, shadow
boundaries, traversing composed trees, distribution, template
documents, custom elements, registering elements, element registries,
element factories, shadow roots, etc.

Remove the following features from the IDLs and from the binding
generators: CustomElementCallbacks, Reflect*, EventHandler.

Remove the CSS custom pseudo-element concept, since we no longer have
a UA style sheet worth talking about, no longer have shadow trees or
custom elements, no longer use pseudo-elements, and generally
therefore don't use this code at all.

Maybe this will work now that we have less code.

Update to mojo 5561fc7ae1717f8013a873f744fdae116d046e74

Use "org.domokit" in Sky shell iOS

Roll mojo

Updated CONTRIBUTING.md

Draw debug rects when flex overflows and debugPaintSizeEnabled

Conflicts:
	sky/sdk/lib/rendering/flex.dart

These directories aren't referenced anymore.

Support for non-alphabetic baselines

Remove //third_party/protobuf

This code isn't referenced anymore.

Remove //mojom

I didn't mean to check these in.

No one references this code.

Remove some unnessary dependencies

- angle
- boringssl
- smhasher
- cython

Remove these dependencies required stripping down some BUILD.gn files to remove
targets we don't build.

BUG=
R=abarth@chromium.org

Review URL: https://codereview.chromium.org/1245683003 .

Still experimenting to see if we can consistently get green results from Travis.

Improve the READMEs further

Improve developer-facing READMEs

Also, stop pushing the SDK to sky_sdk now that this repo exists.

Experiment with building Android on Travis

Previously, we were using too many parallel instances of $CXX, which caused
crashes. Now we use -j 2, which seems to work. Also, use fewer processes during
testing.