Docs: recommend "-no-window" emulator switch

This changes the docs to recommend the "-no-window" flag over a
combination of "xvfb-run" + "-gpu off". I originally recommended the
latter, because ctrl-C didn't close the emulator when I used
"-no-window" (see internal bug http://b/127373262).

I'm changing my recommendation because:

 1. I tried "-no-window" again recently, and can't reproduce the problem
    from before
 2. "-gpu off" triggers a crash when running gradle tests which use
    ActivityTestRule , while "-no-window" avoids this issue (see
    internal bug http://b/127061659)

Bug: 918221
Test: Upload to gerrit > open file > click "gitiles"
Change-Id: I63fba988885b88b128adcee8cc670c11460ca079
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1560517
Commit-Queue: Peter Wen <[email protected]>
Auto-Submit: Nate Fischer <[email protected]>
Reviewed-by: Peter Wen <[email protected]>
Cr-Commit-Position: refs/heads/master@{#649498}
1 file changed