Add WebGL context version to WebGraphicsContext3D::Attributes

The webGLVersion attribute will be used to toggle validation settings in
the command buffer, so that for example code for replacing incomplete
textures with black can live only in the command buffer, and code
implementing nearly identical rules can be removed from
WebGLRenderingContext. This should help with overhead.

The added webGL attribute will be used to make enabling GLSL WebGL
validation in the command buffer cleaner. Now it is enabled based on the
noExtensions flag, which assumes that no other contexts besides WebGL
use the noExtensions flag.

BUG=390827
TEST=WebGL conformance tests

Review URL: https://codereview.chromium.org/372273002

git-svn-id: svn://svn.chromium.org/blink/trunk@177913 bbb929c8-8fbe-4397-9dbb-9b2b20218538