gpu: Use GetUniformSetup computed result size.

R=piman@chromium.org
BUG=468936

Review URL: https://codereview.chromium.org/1016193003

Cr-Commit-Position: refs/heads/master@{#321489}
error::Error GLES2DecoderImpl::HandleDiscardBackbufferCHROMIUM(
    uint32 immediate_data_size,
    const void* cmd_data) {
  if (surface_->DeferDraws())
    return error::kDeferCommandUntilLater;
  if (!surface_->SetBackbufferAllocation(false))
    return error::kLostContext;
  backbuffer_needs_clear_bits_ |= GL_COLOR_BUFFER_BIT;
  backbuffer_needs_clear_bits_ |= GL_DEPTH_BUFFER_BIT;
  backbuffer_needs_clear_bits_ |= GL_STENCIL_BUFFER_BIT;
  return error::kNoError;
}

error::Error GLES2DecoderImpl::HandleDiscardBackbufferCHROMIUM(
    uint32 immediate_data_size,
    const void* cmd_data) {
  if (surface_->DeferDraws())
    return error::kDeferCommandUntilLater;
  if (!surface_->SetBackbufferAllocation(false))
    return error::kLostContext;
  backbuffer_needs_clear_bits_ |= GL_COLOR_BUFFER_BIT;
  backbuffer_needs_clear_bits_ |= GL_DEPTH_BUFFER_BIT;
  backbuffer_needs_clear_bits_ |= GL_STENCIL_BUFFER_BIT;
  return error::kNoError;
}
