[System Web Apps] Extract launch utilities, fix launch disposition.

This CL extracts System Web App launch utilities into its own file as
suggested in https://chromium-review.googlesource.com/c/chromium/src/+/1370239.

It also changes the launch disposition for System Web Apps to CURRENT_TAB to
correctly provide singleton behavior.

Bug: 836128
Change-Id: Iefd691ed3a5c1f1858197599cff5282e22a254ff
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1504603
Reviewed-by: Trent Apted <tapted@chromium.org>
Reviewed-by: Nicholas Verne <nverne@chromium.org>
Reviewed-by: Alexey Baskakov <loyso@chromium.org>
Commit-Queue: calamity <calamity@chromium.org>
Cr-Commit-Position: refs/heads/master@{#641112}
7 files changed