[Spellcheck] Unify dictionary handling.

Remove platform-specific handling for dictionary.

R=rlp@chromium.org
BUG=none

Review URL: https://codereview.chromium.org/121523002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@243978 0039d316-1c4b-4281-b951-d872f2087c98
Revert of https://codereview.chromium.org/127463003/
Reason for revert: Reverting on Canary to maintain port selection there.  Will land on Dev branch.

TBR=wtc@chromium.org,rch@chromium.org
NOTREECHECKS=true
NOTRY=true
BUG=329255

Review URL: https://codereview.chromium.org/132603002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@243979 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 243979