[css-flexbox] Rename some variables for more clarity

- Make it clear that the hypothetical main size is a content size
- Rename innerFlexBaseSize to flexBaseContentSize as that's
  the terminology we usually use

R=eae@chromium.org,dgrogan@chromium.org
TEST=n/a, no change in behavior

Review-Url: https://codereview.chromium.org/2162163002
Cr-Commit-Position: refs/heads/master@{#406392}
1 file changed