Create CrashReportDatabase interface, a test, and a Mac implementation.

util/mac/xattr_test.cc

Index: util/mac/xattr_test.cc
diff --git a/util/mac/xattr_test.cc b/util/mac/xattr_test.cc
index c02aa08c383611c5af39f86eeb748e79d3cc7a92..a0545efe4511bc6a67449688adeaaefc6e28d091 100644
--- a/util/mac/xattr_test.cc
+++ b/util/mac/xattr_test.cc
@@ -55,6 +55,12 @@ class Xattr : public testing::Test {
 
 const char kKey[] = "com.google.crashpad.test";
 
+TEST_F(Xattr, HasXattr) {
+  EXPECT_FALSE(HasXattr(path(), kKey));
+  EXPECT_TRUE(WriteXattr(path(), kKey, ""));
+  EXPECT_TRUE(HasXattr(path(), kKey));
+}
+
 TEST_F(Xattr, ReadNonExistentXattr) {
   std::string value;
   EXPECT_FALSE(ReadXattr(path(), kKey, &value));