Unittests may take compilation dependencies without committing them,
because no code object is ever created. This CL ensures that after each
unittest, all uncommitted dependencies are discarded. Without this,
the compilation dependencies leaks pointers to already freed C++ objects
between test-cases, which are then accessed after free if a unittest
invalidates the corresponding compilation dependency.

Bug: v8:851886
Change-Id: I55d989c42d45f5d713613bc1d3f86e00bd1b8d21
Reviewed-on: https://chromium-review.googlesource.com/1113313
Commit-Queue: Sigurd Schneider <sigurds@chromium.org>
Reviewed-by: Camillo Bruni <cbruni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#53998}