Fix the suppression of libfontconfig.

This library is supposed to be excluded from LSAN, but somehow
we're still seeing leaks from this library [1].

Fix this by excluding the path to the library in third_party. Leave
the existing entry in case it's somehow catching other libfontconfig
leaks.

[1]
https://logs.chromium.org/logs/chromium/buildbucket/cr-buildbucket.appspot.com/8914727183139935872/+/steps/browser_tests__with_patch_/0/logs/Deterministic_failure:_PDFExtensionInternalLinkClickTest.CtrlLeft__status_CRASH_/0

Bug:961017

Change-Id: I8869755987ff595e9df634dc3d997f0eb136cf66
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1614437
Commit-Queue: Alexander Potapenko <glider@chromium.org>
Reviewed-by: Alexander Potapenko <glider@chromium.org>
Reviewed-by: Eric Orth <ericorth@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#663793}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 0fa868445af6bd7a6e3a791172481dafe87ab98c

sanitizers/lsan_suppressions.cc

@@ -21,6 +21,7 @@ char kLSanDefaultSuppressions[] =
 
     // False positives in libfontconfig. http://crbug.com/39050
     "leak:libfontconfig\n"
+    "leak:third_party/fontconfig/*\n"
     // eglibc-2.19/string/strdup.c creates false positive leak errors because of
     // the same reason as crbug.com/39050. The leak error stack trace, when
     // unwind on malloc, includes a call to libfontconfig. But the default stack