Use empty string in local discovery request header

BUG=308327

Review URL: https://codereview.chromium.org/27622002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@229380 0039d316-1c4b-4281-b951-d872f2087c98
Check if GetString's return value is NULL

If we lose a context very early after initialization this call could return
NULL.  Check it and fail the Bind..() call if this occurs.

BUG=305886

Review URL: https://codereview.chromium.org/27644002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@229381 0039d316-1c4b-4281-b951-d872f2087c98
[SyncFS] Split out MetadataDatabase utility

BUG=240165

Review URL: https://codereview.chromium.org/27656002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@229382 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 229381
SVN changes up to revision 229382