Up until now we used int32_t for NodeId, but that was not ideal because
negative values are invalid for NodeId and we use it as an array index
for example in the NodeMarker class, where C++ compilers on x64 have to
generate code that does proper sign extension for the indices, which is
completely unnecessary.

R=svenpanne@chromium.org

Review URL: https://codereview.chromium.org/1178403004

Cr-Commit-Position: refs/heads/master@{#28997}