Add metrics for app banner preventDefault() and prompt().

Add metrics for app banner preventDefault() and prompt().

The beforeinstallprompt event signals an imminent app install banner for
a page. This CL adds UMA stats for preventDefault() being called on the
event (stopping the banner from appearing), and for prompt() being
called on the event (making the banner appear subsequent to
a preventDefault()).

BUG=625716
Review-Url: https://codereview.chromium.org/2122783002
Cr-Commit-Position: refs/heads/master@{#404039}
(cherry picked from commit 4f6382c45e105322038b6fa1885cd1f479ef5ff8)

Committed: https://chromium.googlesource.com/chromium/src/+/b533085ebf0bb4148b47ec27faf62e1619a4c5f1

[dominickn, 2016-07-08 00:48:44]

Description was changed from

==========
Add metrics for app banner preventDefault() and prompt().

The beforeinstallprompt event signals an imminent app install banner for
a page. This CL adds UMA stats for preventDefault() being called on the
event (stopping the banner from appearing), and for prompt() being
called on the event (making the banner appear subsequent to
a preventDefault()).

BUG=625716

Review-Url: https://codereview.chromium.org/2122783002
Cr-Commit-Position: refs/heads/master@{#404039}
(cherry picked from commit 4f6382c45e105322038b6fa1885cd1f479ef5ff8)
==========

to

==========
Add metrics for app banner preventDefault() and prompt().

The beforeinstallprompt event signals an imminent app install banner for
a page. This CL adds UMA stats for preventDefault() being called on the
event (stopping the banner from appearing), and for prompt() being
called on the event (making the banner appear subsequent to
a preventDefault()).

BUG=625716

Review-Url: https://codereview.chromium.org/2122783002
Cr-Commit-Position: refs/heads/master@{#404039}
(cherry picked from commit 4f6382c45e105322038b6fa1885cd1f479ef5ff8)

Committed:
https://chromium.googlesource.com/chromium/src/+/b533085ebf0bb4148b47ec27faf6...
==========

[dominickn, 2016-07-08 00:48:47]

Committed patchset #1 (id:1) manually as
b533085ebf0bb4148b47ec27faf62e1619a4c5f1.