Framebuffer clear() needs to consider the situation some draw buffers are disabled.

This is when we expose DrawBuffers extension.

BUG=376951
TEST=the attached test case, webgl conformance
R=kbr@chromium.org,bajones@chromium.org

Review URL: https://codereview.chromium.org/315283002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@275338 0039d316-1c4b-4281-b951-d872f2087c98
void GLES2DecoderImpl::RenderbufferStorageMultisampleHelper(
    const FeatureInfo* feature_info,
    GLenum target,
    GLsizei samples,
    GLenum internal_format,
    GLsizei width,
    GLsizei height) {
  if (feature_info->feature_flags().is_angle) {
    glRenderbufferStorageMultisampleANGLE(
        target, samples, internal_format, width, height);
  } else if (feature_info->feature_flags().use_core_framebuffer_multisample) {
    glRenderbufferStorageMultisample(
        target, samples, internal_format, width, height);
  } else {
    glRenderbufferStorageMultisampleEXT(
        target, samples, internal_format, width, height);
  }
}

void GLES2DecoderImpl::RenderbufferStorageMultisampleHelper(
    const FeatureInfo* feature_info,
    GLenum target,
    GLsizei samples,
    GLenum internal_format,
    GLsizei width,
    GLsizei height) {
  if (feature_info->feature_flags().is_angle) {
    glRenderbufferStorageMultisampleANGLE(
        target, samples, internal_format, width, height);
  } else if (feature_info->feature_flags().use_core_framebuffer_multisample) {
    glRenderbufferStorageMultisample(
        target, samples, internal_format, width, height);
  } else {
    glRenderbufferStorageMultisampleEXT(
        target, samples, internal_format, width, height);
  }
}
