Forward password-store switch to OSCrypt component

Password manager uses a switch to allow the user to override the
auto-detection of the appropriate password store. OSCrypt should
respect this switch as well.

The switch value is read and passed to OSCrypt at a very early
point in Chrome's start, before any of OSCrypt's dependents
use it.

I also reworked OSCrypt's build to make it simpler for chrome to
deduce whether the linux implementation of OSCrypt will be used.
 - Previously, os_crypt_linux was used only if we also decided to
   link at least one linux backend. Otherwise we used os_crypt_posix.
 + Now, we always use the linux implementation for linux. If no
   KeyStorage is linked, the linux implementation defaults to the
   same behavior as for posix.

BUG=602624

Review-Url: https://codereview.chromium.org/2118443002
Cr-Commit-Position: refs/heads/master@{#405114}
8 files changed