When FLAG_trace_ignition_dispatches is enabled, a dispatch counter is
kept for each pair of source-destination bytecode handlers.

Each counter saturates at max uintptr_t value.

Counters are dumped as a JSON-encoded object of objects, such that
each key on the top level object is a source bytecode name, and each key
on the corresponding value is a destination bytecode name, with the
associated counter as value. The output file name can be controlled
with the FLAG_trace_ignition_dispatches_output_file flag.

The JSON file may be written by calling
Interpreter::WriteDispatchCounters(), which is done for d8 in
Shell::OnExit, if FLAG_trace_ignition_dispatches is enabled.

BUG=v8:4899
LOG=N

Review URL: https://codereview.chromium.org/1828633003

Cr-Commit-Position: refs/heads/master@{#35380}