Wait until a Tab can complete an MPArch activation.

Post the MPArch refactor (crrev.com/c/5961156) a tab can momentarily
have the active page detached and none attached, during an activation.
This CL attempts to avoid seeing this state as an error state, and
instead gives the tab a chance to acquire a new page and complete its
navigation before calling a pending navigation as complete/errored.

Change-Id: I525d5e06a20ec75a8ce2bb0f7ee83359689a77a0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6177606
Commit-Queue: Alex N. Jose <alexnj@chromium.org>
Reviewed-by: Alex Rudenko <alexrudenko@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1412919}