Allowed Uint16 arrays to act as the data source for OES_half_float textures

Addresses spec change made at https://github.com/KhronosGroup/WebGL/pull/475

BUG=365435

Review URL: https://codereview.chromium.org/234133002

git-svn-id: svn://svn.chromium.org/blink/trunk@172553 bbb929c8-8fbe-4397-9dbb-9b2b20218538