libwebp: Add a Build file for GN (from libwebp.gyp)

Android specific build rules, and some other rules not converted yet.

BUG=

Review URL: https://codereview.chromium.org/238523003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@264086 0039d316-1c4b-4281-b951-d872f2087c98
2 files changed