- Remove Declaration::initialization(), move logic into parser.
  The backends should only care about the actual initialization flag on the
  variable.
- Introduce DeclareVariable convenience function that covers most cases of
  variable declarations.

R=adamk@chromium.org
BUG=

Review-Url: https://codereview.chromium.org/2223843002
Cr-Commit-Position: refs/heads/master@{#38477}