Disable OES_texture_float WebGL extension

The following WebGL 1.0.2 conformance tests are causing crashes in
WebGLRenderingContext::texImage2DImpl:

conformance/extensions/oes-texture-float-with-canvas.html
conformance/extensions/oes-texture-float-with-image-data.html
conformance/extensions/oes-texture-float-with-image.html

Turning off the extension to avoid crashing.

Change-Id: Ideaf118cbd4770cd3b1f677bf342d9cc802a6688
CRs-fixed: 427617