Replace FilePath declaration with file_path.h include

Fixes a clang error.  Also use "base::FilePath" since FilePath has been
moved to the base namespace.

BUG=chromium-os:39046
TEST=The following command passes:
USE="chrome_internal" CFLAGS="-clang -print-cmdline"
CXXFLAGS="-clang -print-cmdline" emerge-x86-alex crash-reporter

Change-Id: I7da82093c5685b5b556cba971b1e1b14ac0f59c4
Signed-off-by: Simon Que <sque@chromium.org>
Reviewed-on: https://gerrit.chromium.org/gerrit/43577
Reviewed-by: Ben Chan <benchan@chromium.org>
diff --git a/crash_reporter/user_collector.cc b/crash_reporter/user_collector.cc
index 26c7dc1..a89ea81 100644
--- a/crash_reporter/user_collector.cc
+++ b/crash_reporter/user_collector.cc
@@ -53,6 +53,8 @@
 const char *UserCollector::kUserId = "Uid:\t";
 const char *UserCollector::kGroupId = "Gid:\t";
 
+using base::FilePath;
+
 UserCollector::UserCollector()
     : generate_diagnostics_(false),
       core_pattern_file_(kCorePatternFile),