Standardize parameters passed when Tabs are created asynchronously.
* Pulls out PendingDocumentData from the DocumentTabModelSelector and makes
it a standalone thing. The new AsyncTabCreationParamsManager acts as the
DocumentTabModelSelector did and stores a mapping of Tab IDs to pending
data, which are now stored in AsyncTabCreationParams.
* Callsites are all changed to use the new AsyncTabCreationParams instead.
* AsyncTabCreationParams now stores a LoadUrlParams instead of random fields
from the LoadUrlParams.
* Fields in the AsyncTabCreationParams are clearly marked exclusive, based
on how they are constructed.
BUG=508333
Review URL: https://codereview.chromium.org/
1235523003
Cr-Commit-Position: refs/heads/master@{#338411}