The --log-file switch lets users specify the logging location. It was
added as a generic flag and implemented in app_shell in
crrev.com/c/647314.

This CL expands the switch to other logging configurers, notably
including chrome and content_shell.

In Chrome, --log-file=<PATH> will override the $CHROME_LOG_FILE
environmental variable if both are present.

Bug: 760431
Change-Id: I72448b1a8cdea5b5818badd340594a2f9d6d8cd2
Reviewed-on: https://chromium-review.googlesource.com/685997


Reviewed-by: Jochen Eisinger <jochen@chromium.org>
Commit-Queue: Michael Giuffrida <michaelpg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#505287}