Support default values for POD struct fields in mojom IDL.

TODO: default values for arrays and structs.

BUG=320082

Review URL: https://codereview.chromium.org/138143003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@246424 0039d316-1c4b-4281-b951-d872f2087c98
Reland r246322 - Only install extensions into the main world

BUG=none
TBR=dcarney@chromium.org

Review URL: https://codereview.chromium.org/144023012

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@246425 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 246425