Fix import of trace event files where B/E events' args don't match

[jlfwong]

In #273, I changed CallTreeProfileBuilder.leaveFrame to fail hard when you request to leave a frame different from the one at the top of the stack. It turns out we were intentionally doing this for trace event imports, because args are part of the frame key, and we want to allow profiles to be imported where the "B" and "E" events have differing args field.

This PR fixes the import code to permissively allow the "args" field to not match between the "B" and "E" fields.

A note on intentional differences between speedscope and chrome://tracing

chrome://tracing will close whichever frame is at the top when it gets an "E" event, regardless of whether the name or the args match. speedscope will ignore the event entirely if the "name" field doesn't match, but will warn but still close the frame if the "name"s match but the "args" don't.

[
  {"pid": 0, "tid": 0, "ph": "B", "name": "alpha", "ts": 0},
  {"pid": 0, "tid": 0, "ph": "B", "name": "beta", "ts": 1},
  {"pid": 0, "tid": 0, "ph": "E", "name": "gamma", "ts": 2},
  {"pid": 0, "tid": 0, "ph": "E", "name": "beta", "ts": 9},
  {"pid": 0, "tid": 0, "ph": "E", "name": "alpha", "ts": 10}
]

speedscope

warning: ts=2: Request to end "gamma" when "beta" was on the top of the stack. Doing nothing instead.

chrome://tracing

[coveralls]

Coverage increased (+0.3%) to 47.838% when pulling b2579aa on jlfwong/trace-event-mismatch-leniency into aee2dfd on master.