With the recent changes to the incremental marking API we can now kick off
incremental marking while respecting callback flags.

Performance neutral for smoothness.image_decoding_cases on N9 (read: does not
crash) as long as we synchronously process phantom callbacks
(kGCCallbackFlagForced).

OORT single run:
  "marksweep": {
    "count": 5,
    "pause_min": 7.5,
    "pause_max": 158.8,
    "pause_avg": 97.52000000000001,
    "pause_gt_10ms": 4
  }
  --- vs ---
  "marksweep": {
    "count": 5,
    "pause_min": 16.2,
    "pause_max": 22.1,
    "pause_avg": 19.32,
    "pause_gt_10ms": 5
  }

The number of actual full GCs varies. The improvement manifests in reduced
maximum and average pauses.

BUG=chromium:515795
LOG=N

Review URL: https://codereview.chromium.org/1271253002

Cr-Commit-Position: refs/heads/master@{#30028}