gpu: support GL_TEXTURE_CUBE_MAP destination target to Copy(Sub)TextureCHROMIUM.

It makes it possible to optimize WebGL.

TEST=gl_tests GLCopyTextureCHROMIUMTests/GLCopyTextureCHROMIUMTest.*
BUG=517548

Review URL: https://codereview.chromium.org/1275773003

Cr-Commit-Position: refs/heads/master@{#347354}
9 files changed