A bug in error printing meant that we failed to do proper type checks
before calling into C++ code, which could lead to RUNTIME_ASSERT
failures if methods are called on alternative receivers. This patch
adds the right type checks.

BUG=chromium:596718
LOG=Y
R=adamk

Review URL: https://codereview.chromium.org/1831053003

Cr-Commit-Position: refs/heads/master@{#35069}