Since Ignition dispatch counters have been made accessible from
JavaScript via getIgnitionDispatchCounters() in [1], writing
them to a file at the end of the execution does not seem the best
default anymore.

Following this commit, a file is written only if d8 is invoked
with --trace-ignition-dispatches-output-file.

[1] https://crrev.com/905becd13b8696e126255decf130fdb9e1d9aa30

LOG=N
BUG=v8:4899

Review-Url: https://codereview.chromium.org/1943923002
Cr-Commit-Position: refs/heads/master@{#36015}