After thinking more about this I'm afraid we'd potentially leave users in a bad state.  In particular by the time we return this error we would have already created the new ext4-encrypted directory, which means that the user can't log in to their profile to clear up space.

A few other options:
 - Expose a method over dbus to get the minimum required space so that the UI can check using the correct amount
 - Calculate a more accurate overhead estimate of the main thread
  - Doesn't solve the problem, just reduces probability
  - We currently assume a worst case scenario of all directories duplicated using the size they take in ecryptfs.  In reality directories use more space on ecryptfs (due to the longer encrypted file names) and we will only have a subset duplicated at any given time.

One way to clear up space without logging in to a profile is evicting (automatically, or after asking the user) cache files.
Also, we can remove profiles which haven't been touched for long when there are many profile on a device (which can happen frequently in school classes).

The cleanup code that does this is already run at boot time (on low disk situations) before the crypto migration, so we should have all this low-hanging fruit cleared already.  It is possible that the user booted and first logged in to (and filled up) one profile then switched to another to migrate, in which case this could be helpful.

After talking with fukino@ we decided that raising the minimum space requirements in the UI to 50MB should cover almost all cases (up to about 10k folders) with minimal effort.  There's still the possibility of a user have an excessive number of folders hitting this and getting their profile wiped, but it should be extremely rare.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/55067bd6ae2557dd1ca2ef1dcf796fe7d2175456

commit 55067bd6ae2557dd1ca2ef1dcf796fe7d2175456
Author: dspaid <dspaid@chromium.org>
Date: Mon Jul 03 04:33:55 2017

Update minimum space for migration to 50MB

Users with large numbers of directories may require more space overhead
as any given directory tree may be duplicated on both source and
destination.  Increase the minimum migration storage space to account
for this.

BUG= 737343 
TEST=Fill disk and attempt to migrate

Review-Url: https://codereview.chromium.org/2968643002
Cr-Commit-Position: refs/heads/master@{#483940}

[modify] https://crrev.com/55067bd6ae2557dd1ca2ef1dcf796fe7d2175456/chrome/browser/chromeos/arc/arc_migration_constants.h

Is this Fixed? 
Any need to merge this ? 

No need to merge, M61 is fine.  No additional work needed.