Clean up FreeType code for 2.3.8.

We already require FreeType 2.3.8 in order to compile and run.
Make this requirement explicit and take advantage of it.

Review URL: https://codereview.chromium.org/748063003
Add -ftrapv to debug builds.

This adds instrumentation to check that signed integers never under- or
overflow, which is undefined behavior.

BUG=skia:3096

Review URL: https://codereview.chromium.org/702893003
Add pop_back to GrTRecorder.h

Review URL: https://codereview.chromium.org/750613002
Revert of Add -ftrapv to debug builds. (patchset #1 id:1 of https://codereview.chromium.org/702893003/)

Reason for revert:
Not clean yet.

Original issue's description:
> Add -ftrapv to debug builds.
>
> This adds instrumentation to check that signed integers never under- or
> overflow, which is undefined behavior.
>
> BUG=skia:3096
>
> Committed: https://skia.googlesource.com/skia/+/ebacb6127a952910f43a59460af43427d93df46c

TBR=bungeman@google.com,mtklein@chromium.org
NOTREECHECKS=true
NOTRY=true
BUG=skia:3096

Review URL: https://codereview.chromium.org/751713002