Add assert when things go wrong.
Add assert when things go wrong.

This helps diagnosing user error.

R=luqui@chromium.org
BUG=

Review URL: https://codereview.chromium.org/1528323006

git-svn-id: 239fca9b83025a0b6f823aeeca02ba5be3d9fd76@298075 0039d316-1c4b-4281-b951-d872f2087c98

eunchong/build,eunchong/build,eunchong/build,eunchong/build