* GC's flags are now proper flags and not int.
* Callback flags are not threaded through but only set once like gc flags
* Callers of methods that trigger GCs need to pass a reason when not using
  the default parameters.

Furthermore, each GC invocation can be passed the GC and GCCallback flags. We
usually override the currently set flags upon finishing a GC cylce, but are able
to restore the previously set if desired. This is useful for explicitely
triggered scavenges or external requests that interrupt the current behaviour.

BUG=

Review URL: https://codereview.chromium.org/1314863003

Cr-Commit-Position: refs/heads/master@{#30457}