This CL changes the inline ScopedTempDir::path() to an out-of-line
GetPath() and enhances it with a DCHECK against the directory not
having been created yet.

FAQ

Q: Why GetPath?
A: Because of the added DCHECK in the path accessor, which needs
   #include "base/logging.h", there were two alternatives:
    (A) Keep the inline path() and #include "base/logging.h" in the header.
    (B) Change the inline path() to an out-of-line GetPath and implement it
        in the .cc file.

   (A) would affect compile-time, (B) affects run-time of the tests. I made no
   measuring, but obtaining the path is usually a one-off step in a test, so an
   unlikely hotspot. Therefore the trade-off seems much better for (B).

Q: Why deprecating path() instead of deleting it?
A: Due to the big amount of callsites (almost 600 files in the whole codebase),
   this change is rolled out in phases. This CL changes //base. After all of the
   codebase is converted, the deprecated path() will be deleted.

R=danakj@chromium.org
BUG=640599

Review-Url: https://codereview.chromium.org/2275553005
Cr-Commit-Position: refs/heads/master@{#417241}