[Presentation API] Delete StartSession queueing logic.
Per the API spec, the browser may choose to reject an incoming
StartSession request if there is already one in progress, due to the
call opening a dialog modal to the tab. This patch removes the incorrect
queueing logic that is currently in PresentationServiceImpl. It changes
StartSession so that is if there is a pending StartSession callback,
subsequent StartSession requests will be immediately rejected.
Note that PresentationServiceDelegateImpl will also invoke the error
callback immediately if the dialog controller cannot create a new
dialog for StartSession (because there is already one open).
BUG=501439
Review URL: https://codereview.chromium.org/
1180943008
Cr-Commit-Position: refs/heads/master@{#335269}