This flag is enabled by default for Mac [1][2] and disabled/unavailable 
elsewhere. It's used here [3]. I think it's safe to nuke it out, 
ccameron@ WDYT?

[1] https://cs.chromium.org/chromium/src/content/public/common/content_features.cc?type=cs&sq=package:chromium&g=0&l=108
[2] https://cs.chromium.org/chromium/src/chrome/browser/about_flags.cc?type=cs&sq=package:chromium&g=0&l=1287
[3] https://cs.chromium.org/chromium/src/content/child/runtime_features.cc?type=cs&q=kCanvas2DImageChromium&g=0&l=186

Might be a good idea to change it to a base::Feature -- or is that not really helping?

A base::Feature is fine for now, but features are in my sights in the future :) the real goal here is to reduce the amount of runtime configurations chromium has, so if you reduced this to:

bool ShouldDoTheThing() {
#if defined(OS_MACOSX)
  return true;
#else
  return false;
#endif
}

that would be a-ok in my book.

It can be helpful to be able to disable this feature for debugging. So I'm OK with removing the "enable" flag.

#6: But do you then want to add a corresponding "disable" flag? If so, how long would we retain the flag for?

ccameron: what should we do here? (open question in #7)