This moves the responsibility for printing skip messages to the engine,
rather than the reporter. Reporters now just need to choose how to
display the messages produced by the engine.

This lays the groundwork for #394, since skip messages can now be
emitted during the run of a test.

R=kevmoo@google.com

Review URL: https://codereview.chromium.org//2091173002 .

This makes it possible for a test to be marked skip based on its runtime
behavior rather than being strictly based on its metadata.

See #393 and #394

R=kevmoo@google.com

Review URL: https://codereview.chromium.org//2099503002 .

Closes #420

Review URL: https://codereview.chromium.org//2066113002 .

This adds an event that prints the total number of suites that will be
loaded, and includes the number of tests to be run in each group. This
enables a monotonic (if not entirely accurate) progress bar.

Closes #366

R=kevmoo@google.com

Review URL: https://codereview.chromium.org//1641353002 .

Closes #380

R=alexander.doroshko@gmail.com, kevmoo@google.com

Review URL: https://codereview.chromium.org//1637193003 .

This adds missing Test.groupIDs and PrintEvent.testID fields, and adds a
description to clarify the purpose of FutureEvent. It also now verifies
that the protocol version is compatible.

R=kevmoo@google.com

Review URL: https://codereview.chromium.org//1487753003 .

R=kevmoo@google.com

Review URL: https://codereview.chromium.org//1469863005 .

Closes #11

R=kevmoo@google.com

Review URL: https://codereview.chromium.org//1461293005 .