Refactor MidpointState

The following inline functions were used only with a mandatory LineMidpointState argument:
- startIgnoringSpaces
- stopIgnoringSpaces
- ensureLineBoxInsideIgnoredSpaces
- ensureCharacterGetsLineBox
- checkMidpoints
- deprecatedAddMidpoint.

TrailingObjects class uses some these functions. Since they're inline in BreakingContextInlineHeaders.h,
it blocks separating TrailingObjects into it's own file.

I made these functions as a member of LineMidpointState, and I also updated the call sites.

BUG=334765

Review URL: https://codereview.chromium.org/139933002

git-svn-id: svn://svn.chromium.org/blink/trunk@165177 bbb929c8-8fbe-4397-9dbb-9b2b20218538