When a result of |string.c_str()| expression is passed to a function that takes
a StringPiece, it will measure the length of the string using strlen() function.
This is useless operation because we know the length already.

No behaviour changes.

BUG=679479

Review-Url: https://codereview.chromium.org/2779893005
Cr-Commit-Position: refs/heads/master@{#460501}