Delay deferring resource requests until after first checking cache and extensions

Adds an experimental change to the ResourceScheduler to defer just
before going to network instead of before starting.  This way the
cache and extensions can be checked before deferring.  This is
expected to add cache contention but can potentially reduce PLT
because requests get a head start and reduce SpeedIndex because cached
images can be displayed earlier.

Depends on a non-existant Finch trial to run, the trial is to be added later.

Design doc:
https://docs.google.com/document/d/1TSI3jwozVB_nueWJxzi8uKbgDfsGZRZqw8tvtD-P1Iw/edit?usp=sharing

BUG=328741

[jkarlin, 2016-02-25 15:41:33]

Description was changed from

==========
Delay resource scheduling decisions until network access.

Moved the ResourceScheduler to operate on requests before they use the
network instead of as soon as they are created.  This gets it out of the
way of the cache and service workers (including the new tab page).

BUG=587507
==========

to

==========
Delay resource scheduling decisions until network access.

Moved the ResourceScheduler to operate on requests before they use the
network instead of as soon as they are created.  This gets it out of the
way of the cache and service workers (including the new tab page).

This is a rebase of https://codereview.chromium.org/1706903003/

BUG=587507
==========

[jkarlin, 2016-02-25 15:59:04]

Description was changed from

==========
Delay resource scheduling decisions until network access.

Moved the ResourceScheduler to operate on requests before they use the
network instead of as soon as they are created.  This gets it out of the
way of the cache and service workers (including the new tab page).

This is a rebase of https://codereview.chromium.org/1706903003/

BUG=587507
==========

to

==========
Adds an experimental change to the ResourceScheduler to defer just
before going to network instead of before starting.  This way the
cache and extensions can be checked before deferring.  This is
expected to add cache contention but can potentially reduce PLT
because requests get a head start and reduce SpeedIndex because cached
images can be displayed earlier.

Depends on a non-existant Finch trial to run, the trial is to be added later.

Design doc:
https://docs.google.com/document/d/1TSI3jwozVB_nueWJxzi8uKbgDfsGZRZqw8tvtD-P1...

BUG=328741
==========