This enables {FLAG_debug_code} by default in debug builds. The advantage
is that generated code contained within the snapshot will contain such
debug code. Before we would only get coverage for these pieces with the
no-snapshot builds, which have a meager coverage. One can still pass the
inverse --no-debug-code flag to ensure generated code remains readable
within debug builds as well.

R=machenbach@chromium.org

Review-Url: https://codereview.chromium.org/2528913002
Cr-Commit-Position: refs/heads/master@{#41451}