[SkTextBlob] Remove incorrect builder assert

At the end of TightRunBounds, glyphPosX cannot exceed the start of the
next run.  But glyphPosY is running ahead of glyphPosX (for
kFull_Positioning) => the glyphPosY assert is incorrect.

Since the two pointers advance in lock-step, there isn't much value in
the glyphPosY assert anyway - we might as well remove it.

BUG=chromium:542643
R=reed@google.com,bungeman@google.com

Review URL: https://codereview.chromium.org/1405463004
2 files changed