For a + b, where both a and b are known strings at compile
time, and the combined length of a and b doesn't overflow
the maximum allowed string length, we can constant-fold the
string concatenation during compilation.

R=yangguo@chromium.org

Review-Url: https://codereview.chromium.org/2713093003
Cr-Commit-Position: refs/heads/master@{#43401}