Support frame events from Chrome tracing

[jkriegshauser]

This issue is to start a discussion around this. The import-chrome tool does a pretty good job at converting a chrome-tracing profile capture, but something that we don't have is the idea of frames. Unfortunately, the chrome-tracing spec doesn't seem to have any mention of frames.

It does, however, mention that v-blank is recorded as an Instant event type. import-chrome currently converts these events to messages. But perhaps we could treat instant events that start with "frame" as being a frame mark?

Thoughts?

[wolfpld]

Looks reasonable.

[wolfpld]

Fixed by PR #459.