[ios] Change startup time metrics to main()

A simple change for opening discussion about UMA for Startup time goal:

appLaunchTime is used (in metrics_mediator.mm) to fill the following
existing metrics:
"Startup.ColdStartWithExternalURLTime"
"Startup.ColdStartWithoutExternalURLTime"

appLaunchTime is set in main_controller, which in the simulator, runs
about 0.09 seconds later than IOSChromeMain::InitStartTime(), the
first call in main().

I couldn't find any use of IOSChromeMain::StartTime() so I retyped it
to a base::TimeTicks and used it as a replacement value in
main_controller.  If there is something/someone currently tracking
the two metrics above, then they will get slightly bigger from now on.
If it is not acceptable, then we can look at creating parallel
Startup metrics.

Bug: 988395
Change-Id: Ie3e7d4d1c0f9931a5f760c6fcd5a06ceaa4a0ceb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1722538
Commit-Queue: David Jean <djean@chromium.org>
Reviewed-by: Mark Cogan <marq@chromium.org>
Reviewed-by: Eugene But <eugenebut@chromium.org>
Cr-Commit-Position: refs/heads/master@{#682205}