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Switch to a uniform object construction that does not differentiate between
mixins and regular garabge collected objects and as a consequence can interrupt
both construction cases with garbage collections.

Object construction now uniformely works as follows:
1. The object under construction is marked so in the HeapObjectHeader.
2. Upon discovering such an object it is delayed in a separate marking worklist.
3. Upon hitting the main atomic pause all such objects are conservatively
   scanned for pointers without using the Trace method.

Special cases:
a. Mixins in construction: The HeapObjectHeader cannot retrived for such
   objects. A default GetTraceDescriptor() implementation returning a sentinel
   marker is used to discover that such objects should be delayed.
b. Upon reaching a safepoint (e.g. no stack), in-construction objects are moved
   to a marking worklist that, similar to the regular marking worklist, allows for
   incremental processing.

Effects:
- No more TLS access for no-GC scope on mixin construction.
- No more memory needed for the no-GC scope marker in mixin classes
- MakeGarbageCollected should require less binary size as implementations can be
  aligned.
- Incremental marking Start and Step operations are safe to be called with stack
  (even though it is preferable to not do so).
- Object construction is safe to be used with a concurrent marker as it is ok
  for any objects (mixins as well as normal ones) to be published to the object
  graph during constructors.

This reverts commit ad406608.

Bug: 911662
Change-Id: I7babc6bac25b446a63783bf2c3d19e9b525d9e9a
Reviewed-on: https://chromium-review.googlesource.com/c/1405991Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#622098}