[heap] Use TimeTicks::Now() also for incremental_marking_start_time_

NotifyIncrementalMarkingStart() was using a different timer in the
default configuration to set incremental_marking_start_time_.

Bug: v8:11801
Change-Id: I1551bcc659d025bf8c46c865f5d2bd429934f628
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2930158


Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Dominik Inführ <dinfuehr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#74878}