This adds a dedicated flag for enabling the BytecodeGraphBuilder. The
intention is to be explicit when this variant is being tested and to
avoid unnecessary overhead in production code for a configuration that
is not yet shipping.

R=rmcilroy@chromium.org

Review-Url: https://codereview.chromium.org/1925123002
Cr-Commit-Position: refs/heads/master@{#35892}