This mounts a tmpfs on top of the /home/chronos/Default directory so that when we create the signin profile, it happens there.
This mounts a tmpfs on top of the /home/chronos/Default directory
so that when we create the signin profile, it happens there.

This has the effect of:
1) Reducing I/O contention when the user signs in (because we no
   longer need to clear out the unused signin profile),
2) Improving security, since the profile is cleared at each ui restart,
   eliminating a vector for generating malicious data.

Measurements to find differences in boot times/login times showed no
significant change as a result of this CL.

BUG=chromium:166540
TEST=ran on device, verified mounted tmpfs, measured performance impact.

Change-Id: I898f32880d22845e15d5a18856a531f5c3270640
Reviewed-on: https://chromium-review.googlesource.com/169912
Tested-by: Greg Spencer <00d393fbe6a7572c0cc4bb2312752f641f7e9813@chromium.org>
Reviewed-by: Chris Masone <98e3d65c6b0e9e2391eb4c2e8b1e572b77e544ed@chromium.org>
Commit-Queue: Greg Spencer <00d393fbe6a7572c0cc4bb2312752f641f7e9813@chromium.org>

chadversary/chromiumos.platform.login_manager,chadversary/chromiumos.platform.login_manager,chadversary/chromiumos.platform.login_manager