Fix GN pkg_config with non-ASCII paths

If an include path returned by pkg-config.py contains non-ASCII
characters, such as "🔵", it is JSON-escaped. For example:

    "/home/strager/Projects/chromium/\ud83d\udd35/src/build/linux/debian_bullseye_amd64-sysroot/usr/include/dbus-1.0"
         UTF-16 code units for "🔵": ^^^^^^^^^^^^

These escape sequences are interpreted incorrectly by GN. This leads to
incorrect compiler flags which refer to the wrong directory.

Fix pkg_config's parsing of the values to correctly decode JSON matching
pkg-config.py's JSON output.

cloned into ~/Projects/chromium/🔵/. Then, configure and build Chromium
using defaults per

Test: On Linux, clone Chromium into a directory containing emoji. I
Change-Id: If9f1df9dd517bbb07df4f6c3571185c7e832965a
https: //chromium.googlesource.com/chromium/src/+/main/docs/linux/build_instructions.md.
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4707805
Reviewed-by: Thomas Anderson <thomasanderson@chromium.org>
Commit-Queue: Thomas Anderson <thomasanderson@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1174289}