- Reduce ExpressionResultScope size by reusing accessing the generator
  from its RegisterAllocationScope member
- Mark the ExpressionResultScope destructor non-virtual since there are
  no destructors in subclasses

Change-Id: I3abe1856c091100217c668cd6bd5e7fd96eb8397
Reviewed-on: https://chromium-review.googlesource.com/c/1386485Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Commit-Queue: Camillo Bruni <cbruni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#58398}