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

util/mac/xattr.h

Index: util/mac/xattr.h
diff --git a/util/mac/xattr.h b/util/mac/xattr.h
index b816f3909b565d4ca7346f13fe7d511287b3b3b5..76d8cd1e44561d2457a74fdf1d92abf0d7fb07c1 100644
--- a/util/mac/xattr.h
+++ b/util/mac/xattr.h
@@ -24,6 +24,14 @@
 
 namespace crashpad {
 
+//! \brief Tests if an extended attribute is present on a file.
+//!
+//! \param[in] file The path to the file.
+//! \param[in] name The name of the extended attribute to test.
+//!
+//! \return `true` if the attribute exists, `false` if it does not.
+bool HasXattr(const base::FilePath& file, const base::StringPiece& name);
+
 //! \brief Reads an extended attribute on a file.
 //!
 //! \param[in] file The path to the file.