- Remove semispace target capacity: It's unused and adds some unneeded
  complexity

- Enforcing decl order for SemiSpace
- Move forward declarations in spaces.h to top
- Add all members to default constructor

BUG=chromium:581076
LOG=N

Review URL: https://codereview.chromium.org/1631713002

Cr-Commit-Position: refs/heads/master@{#33515}