See related CL: https://codereview.chromium.org/2111073003/

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/4aaa1c3f03e522bb84b99e6bdf36a9b334865547

commit 4aaa1c3f03e522bb84b99e6bdf36a9b334865547
Author: ryansturm <ryansturm@chromium.org>
Date: Wed Jul 13 20:35:23 2016

Changing the TimeDeltas in DRPPageLoadTiming to Optional

To match the TimeDeltas in page_load_metrics, the
DataReductionProxyPageLoadTiming TimeDeltas are changed to
base::Optional TimeDeltas. Functionally, this will only change the case
where the TimeDelta is zero.

The constructor for DataReductionProxyPageLoadTiming is moved to a new
cc file due to the templated variables creating a complex constructor.

BUG= 626040 

Review-Url: https://codereview.chromium.org/2145743002
Cr-Commit-Position: refs/heads/master@{#405279}

[modify] https://crrev.com/4aaa1c3f03e522bb84b99e6bdf36a9b334865547/chrome/browser/page_load_metrics/observers/data_reduction_proxy_metrics_observer.cc
[modify] https://crrev.com/4aaa1c3f03e522bb84b99e6bdf36a9b334865547/chrome/browser/page_load_metrics/observers/data_reduction_proxy_metrics_observer_unittest.cc
[modify] https://crrev.com/4aaa1c3f03e522bb84b99e6bdf36a9b334865547/components/data_reduction_proxy.gypi
[modify] https://crrev.com/4aaa1c3f03e522bb84b99e6bdf36a9b334865547/components/data_reduction_proxy/core/browser/data_reduction_proxy_pingback_client.cc
[modify] https://crrev.com/4aaa1c3f03e522bb84b99e6bdf36a9b334865547/components/data_reduction_proxy/core/browser/data_reduction_proxy_pingback_client_unittest.cc
[modify] https://crrev.com/4aaa1c3f03e522bb84b99e6bdf36a9b334865547/components/data_reduction_proxy/core/common/BUILD.gn
[add] https://crrev.com/4aaa1c3f03e522bb84b99e6bdf36a9b334865547/components/data_reduction_proxy/core/common/data_reduction_proxy_page_load_timing.cc
[modify] https://crrev.com/4aaa1c3f03e522bb84b99e6bdf36a9b334865547/components/data_reduction_proxy/core/common/data_reduction_proxy_page_load_timing.h