The code will now open files in the browser process before passing the
handles across IPC to the utility process. The utility process in turn
invokes courgette/bsdiff, which memory maps the files and operates on
them as before.

There is a behavioral difference when using the courgette or
courgette_mini tools: the output file will now be created/overwritten
at the start of the operation, and in the case of a failure, will be
deleted. Previously, the output file was created late in the operation
operation and several failure modes would leave it unmodified.

BUG=660325

Review-Url: https://codereview.chromium.org/2534873005
Cr-Commit-Position: refs/heads/master@{#437669}