This commit introduces IgnitionStatisticsExtension, which provides
methods for accessing Ignition statistics and counters from JavaScript.

The extension is registered when FLAG_ignition and
FLAG_trace_ignition_dispatches are both enabled.

For the moment, the only exposed function is
getIgnitionDispatchCounters(), which allows to retrieve Ignition
dispatch counters as a JavaScript object.

BUG=v8:4899
LOG=N

Review URL: https://codereview.chromium.org/1899133004

Cr-Commit-Position: refs/heads/master@{#35816}