Split out DataReductionProxyEventCreator from DataReductionProxyEventStore.
Currently, the storage portion lives on the UI thread (to be read from
net_internals_ui.cc), but all calls for adding events occurs on the IO
thread. In order to maintain thread safety, it used to PostTask to
itself, but that introduces a lifetime problem per bug 472290.
The solution is to split the IO and UI thread portions (in this case
event collection and event storage) and use the existing DRP IO and
DRP Service classes to ensure posted tasks respect the lifetime of
the objects.
BUG=472290
Review URL: https://codereview.chromium.org/
1075283004
Cr-Commit-Position: refs/heads/master@{#326396}