Updated CONTRIBUTING.md

Draw debug rects when flex overflows and debugPaintSizeEnabled

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

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.

Track overflow during flex layout and fix stocks row

R=hixie

Rebaseline stocks test to account for transitions

Remove editing code from the C++ side of Sky.

Also, turn on testing in Travis.

Fix build issues related to symlink.py

There are actually two uses cases for symlink.py. This CL restores
//build/symlink.py to match the version in Chromium and adds a new version in
////sky/build/symlink.py that works properly for the material-design-icons.

Fix #57 Flex needs to understand baselines

This removes the bulk of core/editing/*.
The following files remain, because they might be useful yet:
  EditingBoundary.h
  FindOptions.h
  htmlediting.cpp
  htmlediting.h
  PlainTextRange.cpp
  PlainTextRange.h
  PositionWithAffinity.cpp
  PositionWithAffinity.h
  RenderedPosition.cpp
  RenderedPosition.h
  TextAffinity.h
  TextGranularity.h
  TextIterator.cpp
  TextIterator.h
  VisiblePosition.cpp
  VisiblePosition.h
  VisibleSelection.cpp
  VisibleSelection.h
  VisibleUnits.cpp
  VisibleUnits.h

In addition to remove obviously editing-related stuff like
"ApplyBlockElementCommand.cpp" and "InsertLineBreakCommand.cpp", this
also removes the DOM side of selection, all the caret management and
painting code, composition support (IME) including the relevant
events, spelling checker support, and the undo stack.

Outside the core/editing/* directory, I also deleted the EditorClient,
SpellCheckerClient, and EmptyClients classes. The other changes
outside of editing/ are mostly just about removing mentions of the
selection or carets.

I tried to leave the code for _painting_ selections and composition
runs, though that code is mostly disconnected now.

Add another missing dependency