Pull Mojo SDK in via deps instead of checked in files

This means that //mojo/public is pulled in by gclient using the hash
specified in the DEPS file from the github.com/domokit/mojo_sdk mirror.
Each mojo_sdk commit corresponds to its originating
github.com/domokit/mojo commit and has the origin commit in the
message. Thus to find the mojo_sdk revision to use for a particular
mojo revision, grep through the commit log in the mojo_sdk repo for the
matching "Cr-Mirrored-Commit:" line.

If you need to back your local checkout to earlier than this commit
after running gclient, do 'rm -rf mojo/public && git reset --hard' to
get the locally checked in versions back.  Otherwise you'll get subtree
commits in mojo/public/... and have a bad time.

.gitignore

@@ -25,6 +25,7 @@ Thumbs.db
 /buildtools/
 /dart/
 /dart-pub-cache/
+/mojo/public/
 /native_client/
 /out/
 /out_*/

DEPS

@@ -19,6 +19,7 @@
 
 vars = {
   'chromium_git': 'https://chromium.googlesource.com',
+  'mojo_sdk_revision': '9a5f81a3d2a6d027677366edb5de3ae85f6dbf16',
   'skia_revision': '29ccdf86ab0a1649fd775c9431891bacb1391e99',
   'dart_revision': '95c951ad190f156eb61b99203c2e4948211c44a7',
   'dart_observatory_packages_revision': 'cdc4b3d4c15b9c0c8e7702dff127b440afbb7485',
@@ -53,6 +54,10 @@ deps = {
    '/external/github.com/dart-lang/observatory_pub_packages' + '@' +
    Var('dart_observatory_packages_revision'),
 
+  'src/mojo/public':
+   Var('chromium_git') + '/external/github.com/domokit/mojo_sdk' + '@' +
+   Var('mojo_sdk_revision'),
+
   'src/third_party/skia':
    Var('chromium_git') + '/skia.git' + '@' +  Var('skia_revision'),