Implicitly escape all output that is passed to Log::MessageBuilder.
We escape non-printable characters and the log field separator ','
using the \x00 and \u0000 escape sequences.

Example:
Before: event-foo,"space: ","comma: ,","double quotes: """
After:  event-foo,space: ,comma: \x2C,double quotes: "

This might slightly impact human readability of the log files in
extreme cases. However, most strings do not contain any escaped
characters.

Bug: 
Change-Id: Ic78f6d9932367d02f9f3c3f70b41b5c283bdf880
Reviewed-on: https://chromium-review.googlesource.com/728332
Commit-Queue: Camillo Bruni <cbruni@chromium.org>
Reviewed-by: Adam Klein <adamk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48805}