Labelling with canary milestone.

Is this Chrome code that's being executed, or just code that Chrome is causing to be executed? If it's Chrome code, it seems like we could just instrument this with a toplevel trace span, right?

Both. What is a toplevel trace span?

I think this might be as simple as adding a trace event to -[NSApp sendEvent:] or some such.

From a tracing perspective, a toplevel trace span is just a trace event with the "toplevel" category, like this one (https://cs.chromium.org/chromium/src/ipc/ipc_channel_mojo.cc?q=trace.*toplevel&sq=package:chromium&l=385&dr=C).

From a conceptual standpoint, a toplevel span should be active any time that a Chrome process is active, and needs to wrap any entry points where Chrome code can start running, like this one. This ensures that metrics like the CPU time metric are accurate.

The classic "toplevel" event is MessageLoop::RunTask (https://cs.chromium.org/chromium/src/base/message_loop/message_loop.cc?q=messageloop::runtask&sq=package:chromium&l=409&dr=C), but it looks like that uses  a special trace macro just for it in order to make sure we know where it got called from. I think for our purposes, just using a regular old trace event like the first one I linked to.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/ad0b46843bbcca3ef126402974d9ceaaf136bef5

commit ad0b46843bbcca3ef126402974d9ceaaf136bef5
Author: erikchen <erikchen@chromium.org>
Date: Thu Dec 08 20:58:04 2016

mac: Add tracing for -[BrowserCrApplication sendEvent:].

This captures work triggered from NSEvents for tracing.

BUG= 669116 

Review-Url: https://codereview.chromium.org/2566443002
Cr-Commit-Position: refs/heads/master@{#437334}

[modify] https://crrev.com/ad0b46843bbcca3ef126402974d9ceaaf136bef5/chrome/browser/chrome_browser_application_mac.mm