[Chromecast] Replace 'fontconfig' GN target with dynamic dependency.

Chromecast needs to be able to optionally use //third_party/fontconfig
instead of system headers. This CL acheives parity with the GYP build
for this feature:
https://code.google.com/p/chromium/codesearch#chromium/src/build/linux/system.gyp&l=748

BUG=516899

Review URL: https://codereview.chromium.org/1366653003

Cr-Commit-Position: refs/heads/master@{#350711}
12 files changed