Fix C++ header and source syntax highlighting

cpp and hpp files were sometimes pulling up C mode and not the
extended C++ mode.  This prevented keywords like "class" from being
colored by the highlighter.

Correct the mapping for cpp and hpp to match the MIME string used
by CodeMirror's meta.js, which is how the client UI maps from the
server JSON onto specific modes.

Change-Id: I315844c5bc791338995d93d8200574eaa9005de2

gerrit-server/src/main/resources/com/google/gerrit/server/mime/mime-types.properties

@@ -5,6 +5,7 @@ clj = text/x-clojure
 cl = text/x-common-lisp
 coffee = text/x-coffeescript
 cs = text/x-csharp
+cpp = text/x-c++src
 cxx = text/x-c++src
 d = text/x-d
 dart = application/dart
@@ -20,7 +21,8 @@ glsl = x-shader/x-vertex
 go = text/x-go
 groovy = text/x-groovy
 hs = text/x-haskell
-hxx = text/x-c++hdr
+hpp = text/x-c++src
+hxx = text/x-c++src
 lisp = text/x-common-lisp
 lsp = text/x-common-lisp
 lua = text/x-lua