First cut at UI for saving net_logs data into a temporary file on

First cut at UI for saving net_logs data into a temporary file on
Mobile (Android and iOS). 

Added a new page chrome://net-export to start/stop/send the net_log
for mobile platforms.

Button "Start Saving (after deleting old data)" will start saving to a
temporary file in "NetLog" directory in Temp directory. It deletes all
the old temporary files that were in that directory before creating a
new temporty file. It is a no-op if we are already saving data into a
file.

Button "Stop Saving Data" closes the temporary file so that it could
be mailed. It is a no-op if we haven't started saving data into a
file.

Button "Send Data" emails the data file if a temporary file exists in
"NetLog" directory. It is a no-op if there is no file.

There is only a single temporary file into which we log the
data. NetLog data is saved in the browser process. Logging of net_log
data behaves similar to the command line option --log-net-log.

If chrome://net-export is accessed in multiple tabs, action in first
tab wins. If Stop is clicked in any tab, logging stops. If start is
clicked in any tab, net logging starts.

mmenke@ and/or eroman@ for chrome\browser\resources and webui changes. 
jar@ for net_log_temp_file changes.
jhawkins@chromium.org for chrome\browser\ui\webui and chrome\browser\resources
OWNERS stamp.

R=jar@chromium.org, mmenke@chromium.org
BUG=151212
TEST=chrome://net-export