Make GN write the platform for MSVS GYP files.

This also updates the base GN build to stay in sync with the GYP one.

Review URL: https://codereview.chromium.org/115723003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@242078 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 242078
skip neon code in libwebp when neon is explicitly unavailable

This patch makes it possible to build libwebp without runtime neon detection/usage.

Review URL: https://codereview.chromium.org/95873002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@242079 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 242079