Make sure string constructors are efficient on literals

Separate out the overload when the length is not known, and be sure that
strlen() call is in the header so that strlen("foo") => 3 (since many
compilers support this optimization).

Also delete some unused types.

BUG=pdfium:151
R=thestig@chromium.org

Review URL: https://codereview.chromium.org/1117263004
2 files changed