heap: Use fullpaths in clang plugin ignored/allowed directories

Using just the suffix results in applying the clang plugin to Oilpan
source code if the chromium checkout resides (transitively) under a
directory named "test". In such cases, chromium fails to build.

Bug: 1137252
Change-Id: I6a0e7fb5e175c9f33fe7c79ca7f5790b69eb34ee
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2480569Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Omer Katz <omerkatz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#818383}

tools/clang/blink_gc_plugin/BlinkGCPluginConsumer.cpp

@@ -88,8 +88,10 @@ BlinkGCPluginConsumer::BlinkGCPluginConsumer(
   options_.checked_namespaces.insert("blink");
 
   // Ignore GC implementation files.
-  options_.ignored_directories.push_back("/heap/");
-  options_.allowed_directories.push_back("/test/");
+  options_.ignored_directories.push_back(
+      "third_party/blink/renderer/platform/heap/");
+  options_.allowed_directories.push_back(
+      "third_party/blink/renderer/platform/heap/test/");
 }
 
 void BlinkGCPluginConsumer::HandleTranslationUnit(ASTContext& context) {