Use getters for StyleSurroundData members in ComputedStyle.cpp

In ComputedStyle, we directly access StyleSurroundData members via the
m_surround pointer (e.g. m_surround->m_border), but this assumes that
these members will always be stored in StyleSurroundData. As part of
clockwork, we may change where these members are stored, so this patch
uses getter methods instead, which are independent of where these
members are stored.

Places not affected by this patch:
- Getters/setters, since these will be eventually generated.
- When the surround pointer is used directly (e.g. for fast
  comparisons), since these are unavoidable.
- Modifying variables via m_surround.access(), since they're
  setters not getters. Setters will be refactored in a separate
  patch.

BUG=628043

Review-Url: https://codereview.chromium.org/2785343002
Cr-Commit-Position: refs/heads/master@{#465119}
2 files changed