Add new UMA stats for parallelizable download

Add new UMA stats for parallelizable download

To study the effectiveness of parallel download,
we want to compare the bandwidth if parallel downloads are not parallelized.
Because parallel download has requirements on file size, so comparing with all
other downloads is be fair.
This CL records the UMA metrics for downloads that satisfy the requirement
of parallel download, while don't use parallel requests.

BUG=644352, 712892
TBR=dtrainor@chromium.org,alexmos@chromium.org,isherman@chromium.org

Review-Url: https://codereview.chromium.org/2823273004
Cr-Commit-Position: refs/heads/master@{#466128}
(cherry picked from commit 2269e30d4ea0874704bfbd4021c087ba960e7ea5)

Review-Url: https://codereview.chromium.org/2839693002 .
Cr-Commit-Position: refs/branch-heads/3071@{#168}
Cr-Branched-From: a106f0abbf69dad349d4aaf4bcc4f5d376dd2377-refs/heads/master@{#464641}
Committed: https://chromium.googlesource.com/chromium/src/+/f7823c3140937e3fbfdc8eee48dfd780707b78e3

[qinmin, 2017-04-24 17:44:22]

Description was changed from

==========
Add new UMA stats for parallelizable download

To study the effectiveness of parallel download,
we want to compare the bandwidth if parallel downloads are not parallelized.
Because parallel download has requirements on file size, so comparing with all
other downloads is be fair.
This CL records the UMA metrics for downloads that satisfy the requirement
of parallel download, while don't use parallel requests.

BUG=644352,712892
TBR=dtrainor@chromium.org,alexmos@chromium.org,isherman@chromium.org

Review-Url: https://codereview.chromium.org/2823273004
Cr-Commit-Position: refs/heads/master@{#466128}
(cherry picked from commit 2269e30d4ea0874704bfbd4021c087ba960e7ea5)
==========

to

==========
Add new UMA stats for parallelizable download

To study the effectiveness of parallel download,
we want to compare the bandwidth if parallel downloads are not parallelized.
Because parallel download has requirements on file size, so comparing with all
other downloads is be fair.
This CL records the UMA metrics for downloads that satisfy the requirement
of parallel download, while don't use parallel requests.

BUG=644352,712892
TBR=dtrainor@chromium.org,alexmos@chromium.org,isherman@chromium.org

Review-Url: https://codereview.chromium.org/2823273004
Cr-Commit-Position: refs/heads/master@{#466128}
(cherry picked from commit 2269e30d4ea0874704bfbd4021c087ba960e7ea5)

Review-Url: https://codereview.chromium.org/2839693002 .
Cr-Commit-Position: refs/branch-heads/3071@{#168}
Cr-Branched-From:
a106f0abbf69dad349d4aaf4bcc4f5d376dd2377-refs/heads/master@{#464641}
Committed:
https://chromium.googlesource.com/chromium/src/+/f7823c3140937e3fbfdc8eee48df...
==========

[qinmin, 2017-04-24 17:44:23]

Committed patchset #1 (id:1) manually as
f7823c3140937e3fbfdc8eee48dfd780707b78e3 (presubmit successful).