We were using the c++11 iterator return value from std::map::erase.
Instead use the post-increment operator to keep a valid iterator
after the call to erase, even on older stl.

BUG=angle:773

Change-Id: Ieaa21a2b5a3fcd5ca229a31d40bdf9b8f3283a3c
Reviewed-on: https://chromium-review.googlesource.com/238440Tested-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>