Revert "[FileAPI] Drop unused pre-opened PepperFileSystemHost constructor"

This reverts r228194. I added that constructor 2 weeks ago as a precursor
to CL 26564009. It won't be unused for long.

BUG=177017

Review URL: https://codereview.chromium.org/27192008

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@228607 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 228607
Fix iterator invalidation after erase(it).

Review URL: https://codereview.chromium.org/26183004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@228608 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 228608
SyncFS: WeakPtrFactory should be the last member in SyncEngine


BUG=303818
TEST=n/a (compile)

Review URL: https://codereview.chromium.org/27107002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@228609 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 228609