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@{#387831}
(cherry picked from commit 8dd3c54659c4e99b4ba31254fed0094defd4ffd1)

[wychen, 2016-04-20 19:26:18]

Committed patchset #2 (id:20001) to pending queue manually as
30ced66e903098f5714bf698b783ef961156f6a7.