Add missing BASE_EXPORT to LogErrorNotReached().

This fixes a linker error that was introduced in
https://codereview.chromium.org/1124673004/.

BUG=484283
TBR=danakj@chromium.org

Review URL: https://codereview.chromium.org/1155163002

Cr-Commit-Position: refs/heads/master@{#331354}
1 file changed