Removes the handling of the flag independent. The flag will be removed in a followup.

The patch changes handling of V8::PersistentBase that are set to Weak:
- The Scavenger ignores the flag independent.
- The Scavenger keeps alive anything that is marked as Active.
- The Scavenger is free to drop weak handles of non-Active object if they 
  are otherwise dead.

Active:
- Any JSObject will always be marked Active.
- Any JSApiObject will be marked Active if it has been modified (=has elements, properties, etc.)

CQ_INCLUDE_TRYBOTS=master.tryserver.blink:linux_trusty_blink_rel;master.tryserver.chromium.linux:linux_optional_gpu_tests_rel;master.tryserver.chromium.mac:mac_optional_gpu_tests_rel;master.tryserver.chromium.win:win_optional_gpu_tests_rel;master.tryserver.chromium.android:android_optional_gpu_tests_rel

Bug: chromium:780749
Change-Id: If1f547f2419930ad9400bd3b11bdbf609cb57649
Reviewed-on: https://chromium-review.googlesource.com/741801
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Hannes Payer <hpayer@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49277}