This is needed because the product requirement is that the value of ECT should be computed once per pageload, and the ECT value should remain the same during the duration of the page load. Old design doc: https://docs.google.com/document/u/1/d/178NjryjFzpfM6cbPHu6YGe3nVmS5kIG8Qi6fXEcoUcc/edit?usp=drive_web

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/dd6cda7fa2075ce73445716bf6698479c572979a

commit dd6cda7fa2075ce73445716bf6698479c572979a
Author: tbansal <tbansal@chromium.org>
Date: Mon Aug 08 21:24:03 2016

Update Effective Connection Type only on main frame requests

Currently, the effective connection type (ECT) is computed
every time there is a request that belongs to main frame.
This CL changes it to change it on every main frame request.

BUG= 635575 

Review-Url: https://codereview.chromium.org/2213683003
Cr-Commit-Position: refs/heads/master@{#410464}

[modify] https://crrev.com/dd6cda7fa2075ce73445716bf6698479c572979a/content/browser/loader/DEPS
[modify] https://crrev.com/dd6cda7fa2075ce73445716bf6698479c572979a/content/browser/loader/resource_loader.cc
[modify] https://crrev.com/dd6cda7fa2075ce73445716bf6698479c572979a/content/browser/loader/resource_loader_unittest.cc