third_party/protobuf: Update to HEAD (83d681ee2c)

Rebase protobuf to current HEAD to pull the 3 following changes:
* 4c5d3ed Fix integer overflow in FastUInt32ToBufferLeft
* 4587a3f [arm/gcc] Don't rely on KUSER_HELPERS feature for atomics
* a5a2c1d generic atomicops: Use strong compare_exchange

This change has been made following steps described in
third_party/protobuf/README.chromium file, please refer to it
for further information.

BUG=599051,673488

Review-Url: https://codereview.chromium.org/2495533002
Cr-Commit-Position: refs/heads/master@{#439821}
999 files changed