[LargeIconService] Allow overriding request URL by a field trial param

[LargeIconService] Allow overriding request URL by a field trial param

This CL integrates last minute comments from product on the first
experiment with fetch favicons on demand - for "Articles for you" on the
New Tab Page.

This CL further allows to override to request URL format using a field
trial parameter so that operating that experiment is more flexible.

BUG=705572
Review-Url: https://codereview.chromium.org/2839653002
Cr-Commit-Position: refs/heads/master@{#467051}
(cherry picked from commit 99b05cd8620a446c0e84fefbf28e8db907b44bdc)

Review-Url: https://codereview.chromium.org/2849813002 .
Cr-Commit-Position: refs/branch-heads/3071@{#290}
Cr-Branched-From: a106f0abbf69dad349d4aaf4bcc4f5d376dd2377-refs/heads/master@{#464641}
Committed: https://chromium.googlesource.com/chromium/src/+/258b6493bff7901661c887ac3b08e5d2a83e2ad7

[jkrcal, 2017-04-28 11:40:01]

Description was changed from

==========
[LargeIconService] Allow overriding request URL by a field trial param

This CL integrates last minute comments from product on the first
experiment with fetch favicons on demand - for "Articles for you" on the
New Tab Page.

This CL further allows to override to request URL format using a field
trial parameter so that operating that experiment is more flexible.

BUG=705572

Review-Url: https://codereview.chromium.org/2839653002
Cr-Commit-Position: refs/heads/master@{#467051}
(cherry picked from commit 99b05cd8620a446c0e84fefbf28e8db907b44bdc)
==========

to

==========
[LargeIconService] Allow overriding request URL by a field trial param

This CL integrates last minute comments from product on the first
experiment with fetch favicons on demand - for "Articles for you" on the
New Tab Page.

This CL further allows to override to request URL format using a field
trial parameter so that operating that experiment is more flexible.

BUG=705572

Review-Url: https://codereview.chromium.org/2839653002
Cr-Commit-Position: refs/heads/master@{#467051}
(cherry picked from commit 99b05cd8620a446c0e84fefbf28e8db907b44bdc)

Review-Url: https://codereview.chromium.org/2849813002 .
Cr-Commit-Position: refs/branch-heads/3071@{#290}
Cr-Branched-From:
a106f0abbf69dad349d4aaf4bcc4f5d376dd2377-refs/heads/master@{#464641}
Committed:
https://chromium.googlesource.com/chromium/src/+/258b6493bff7901661c887ac3b08...
==========

[jkrcal, 2017-04-28 11:40:02]

Committed patchset #1 (id:1) manually as
258b6493bff7901661c887ac3b08e5d2a83e2ad7.