[Desktop FRE] Fix incorrect emission of histogram

[Desktop FRE] Fix incorrect emission of histogram

This CL fixes a bug where the Welcome.Win10.NewPromoPageAdded histogram
is emitted for the wrong page--it is meant to track chrome://welcome-win10,
which corresponds to Welcome.Win10.OldPromoPageAdded tracking the old
default browser promo. This CL also addresses a refactoring nit requested
in a prior review.

BUG=688574
Review-Url: https://codereview.chromium.org/2848713002
Cr-Original-Commit-Position: refs/heads/master@{#470218}
Review-Url: https://codereview.chromium.org/2879193004 .
Cr-Commit-Position: refs/branch-heads/3071@{#562}
Cr-Branched-From: a106f0abbf69dad349d4aaf4bcc4f5d376dd2377-refs/heads/master@{#464641}
Committed: https://chromium.googlesource.com/chromium/src/+/248a38b867f44add9113570c34c6eef71090da80

[fdoray, 2017-05-15 18:56:40]

Description was changed from

==========
[Desktop FRE] Fix incorrect emission of histogram

This CL fixes a bug where the Welcome.Win10.NewPromoPageAdded histogram
is emitted for the wrong page--it is meant to track chrome://welcome-win10,
which corresponds to Welcome.Win10.OldPromoPageAdded tracking the old
default browser promo. This CL also addresses a refactoring nit requested
in a prior review.

BUG=688574

Review-Url: https://codereview.chromium.org/2848713002
Cr-Commit-Position: refs/heads/master@{#470218}
(cherry picked from commit 4000f08774959883da8bef366561798e98cb50eb)
==========

to

==========
[Desktop FRE] Fix incorrect emission of histogram

This CL fixes a bug where the Welcome.Win10.NewPromoPageAdded histogram
is emitted for the wrong page--it is meant to track chrome://welcome-win10,
which corresponds to Welcome.Win10.OldPromoPageAdded tracking the old
default browser promo. This CL also addresses a refactoring nit requested
in a prior review.

BUG=688574

Review-Url: https://codereview.chromium.org/2848713002
Cr-Original-Commit-Position: refs/heads/master@{#470218}
Review-Url: https://codereview.chromium.org/2879193004 .
Cr-Commit-Position: refs/branch-heads/3071@{#562}
Cr-Branched-From:
a106f0abbf69dad349d4aaf4bcc4f5d376dd2377-refs/heads/master@{#464641}
Committed:
https://chromium.googlesource.com/chromium/src/+/248a38b867f44add9113570c34c6...
==========

[fdoray, 2017-05-15 18:56:41]

Committed patchset #1 (id:1) manually as
248a38b867f44add9113570c34c6eef71090da80.