Record domains where Reader Mode is prompted or opened

Record domains where Reader Mode is prompted or opened

Use RAPPOR to anonymously record the domains in two cases:
1. Reader Mode panel is prompted
   In AdaBoost heuristic, this means the page is distillable.
2. Reader Mode panel is opened by the user

BUG=602134
Review URL: https://codereview.chromium.org/1873073002

Cr-Commit-Position: refs/heads/master@{#387830}
(cherry picked from commit 388989c6665e2e46749eead442ffc942562d6e7b)

Committed: https://chromium.googlesource.com/chromium/src/+/a968ab314538464420465fa8dfdfaacedaa30b47

[wychen, 2016-04-20 21:18:01]

Description was changed from

==========
Record domains where Reader Mode is prompted or opened

Use RAPPOR to anonymously record the domains in two cases:
1. Reader Mode panel is prompted
   In AdaBoost heuristic, this means the page is distillable.
2. Reader Mode panel is opened by the user

BUG=602134

Review URL: https://codereview.chromium.org/1873073002

Cr-Commit-Position: refs/heads/master@{#387830}
(cherry picked from commit 388989c6665e2e46749eead442ffc942562d6e7b)
==========

to

==========
Record domains where Reader Mode is prompted or opened

Use RAPPOR to anonymously record the domains in two cases:
1. Reader Mode panel is prompted
   In AdaBoost heuristic, this means the page is distillable.
2. Reader Mode panel is opened by the user

BUG=602134

Review URL: https://codereview.chromium.org/1873073002

Cr-Commit-Position: refs/heads/master@{#387830}
(cherry picked from commit 388989c6665e2e46749eead442ffc942562d6e7b)

Committed:
https://chromium.googlesource.com/chromium/src/+/a968ab314538464420465fa8dfdf...
==========

[wychen, 2016-04-20 21:18:02]

Committed patchset #1 (id:1) manually as
a968ab314538464420465fa8dfdfaacedaa30b47 (tree was closed).