On ports where WebGLRenderingContext's back buffer is managed by the DrawingBuffer class (Source/WebCore/platform/graphics/gpu/DrawingBuffer.h), WebGLRenderingContext::getContextAttributes should be implemented in terms of the DrawingBuffer, not GraphicsContext3D. This mismatch is causing conformance test failures on platforms which don't support multisampling, probably among other problems.
After further examination, the DrawingBuffer determines all of its context creation attributes from the GraphicsContext3D. The only flag it determines itself is whether it can support multisampling based on the presence of the framebuffer_blit and framebuffer_multisample extensions. Fixing just the computation of the antialias flag.
Created attachment 192071 [details]
Comment on attachment 192071 [details]
Clearing flags on attachment: 192071
Committed r145159: <http://trac.webkit.org/changeset/145159>
All reviewed patches have been landed. Closing bug.