[Prerender] Remove the "wash" origin in prerender histograms

[Prerender] Remove the "wash" origin in prerender histograms

This origin was introduced at a time where the prerender metrics
were unable to keep track of multiple origins simultaneously.
When several origins were active at once, the origin would be reported
as a "wash".

This limitation no longer exists, and origins now don't interfere with
each other. Moreover, the wash origin is actually harmful now because
it destroys information.

BUG=704868
Review-Url: https://codereview.chromium.org/2769903005
Cr-Commit-Position: refs/heads/master@{#459437}
(cherry picked from commit 9aac56e95a24dcdeec343f906922f06cddfdf5a5)

Review-Url: https://codereview.chromium.org/2775263002 .
Cr-Commit-Position: refs/branch-heads/3029@{#423}
Cr-Branched-From: 939b32ee5ba05c396eef3fd992822fcca9a2e262-refs/heads/master@{#454471}
Committed: https://chromium.googlesource.com/chromium/src/+/0c7714ff66b36ad6bcbfbc74ed44e6e2652c8e5c

[droger, 2017-03-27 11:48:48]

Description was changed from

==========
[Prerender] Remove the "wash" origin in prerender histograms

This origin was introduced at a time where the prerender metrics
were unable to keep track of multiple origins simultaneously.
When several origins were active at once, the origin would be reported
as a "wash".

This limitation no longer exists, and origins now don't interfere with
each other. Moreover, the wash origin is actually harmful now because
it destroys information.

BUG=704868

Review-Url: https://codereview.chromium.org/2769903005
Cr-Commit-Position: refs/heads/master@{#459437}
(cherry picked from commit 9aac56e95a24dcdeec343f906922f06cddfdf5a5)
==========

to

==========
[Prerender] Remove the "wash" origin in prerender histograms

This origin was introduced at a time where the prerender metrics
were unable to keep track of multiple origins simultaneously.
When several origins were active at once, the origin would be reported
as a "wash".

This limitation no longer exists, and origins now don't interfere with
each other. Moreover, the wash origin is actually harmful now because
it destroys information.

BUG=704868

Review-Url: https://codereview.chromium.org/2769903005
Cr-Commit-Position: refs/heads/master@{#459437}
(cherry picked from commit 9aac56e95a24dcdeec343f906922f06cddfdf5a5)

Review-Url: https://codereview.chromium.org/2775263002 .
Cr-Commit-Position: refs/branch-heads/3029@{#423}
Cr-Branched-From:
939b32ee5ba05c396eef3fd992822fcca9a2e262-refs/heads/master@{#454471}
Committed:
https://chromium.googlesource.com/chromium/src/+/0c7714ff66b36ad6bcbfbc74ed44...
==========

[droger, 2017-03-27 11:48:49]

Committed patchset #1 (id:1) manually as
0c7714ff66b36ad6bcbfbc74ed44e6e2652c8e5c.