only enable hole punching for encrypted video

Currently the hole punching code is for all MSE, which is unncessary.
This change makes hole punching code only applicable for encrypted video
Also cleaning up some unused functions in media_source_delegate.cc

Review URL: https://codereview.chromium.org/141363007

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@247273 0039d316-1c4b-4281-b951-d872f2087c98
Add a script to bootstrap GN from source.

For now only works on 64-bit Linux.

BUG=336441
R=brettw@chromium.org

Review URL: https://codereview.chromium.org/145133007

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@247274 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 247273
SVN changes up to revision 247274