Add string_cache override for geckolib based on gecko atoms.

The work in these patches is based on Ms2ger's gecko-atom crate.

ports/geckolib/string_cache/regen_atom_macro.py

@@ -0,0 +1,36 @@
+# This Source Code Form is subject to the terms of the Mozilla Public
+# License, v. 2.0. If a copy of the MPL was not distributed with this
+# file, You can obtain one at http://mozilla.org/MPL/2.0/.
+
+import re
+import sys
+
+if len(sys.argv) != 2:
+    print "usage: ./%s PATH/TO/OBJDIR" % sys.argv[0]
+objdir_path = sys.argv[1]
+
+
+def line_to_atom(line):
+    result = re.match('^GK_ATOM\((.+),\s*"(.*)"\)', line)
+    return (result.group(1), result.group(2))
+
+
+def symbolify(ident):
+    return "_ZN9nsGkAtoms" + str(len(ident)) + ident + "E"
+
+
+with open(objdir_path + "/dist/include/nsGkAtomList.h") as f:
+    lines = [line for line in f.readlines() if line.startswith("GK_ATOM")]
+    atoms = [line_to_atom(line) for line in lines]
+
+with open("atom_macro.rs", "w") as f:
+    f.write("use gecko_bindings::bindings::nsIAtom;\n\n")
+    f.write("use Atom;\n\n")
+    f.write("pub fn unsafe_atom_from_static(ptr: *mut nsIAtom) -> Atom { unsafe { Atom::from_static(ptr) } }\n\n")
+    for atom in atoms:
+        f.write('extern { pub static %s: *mut nsIAtom; }\n' % symbolify(atom[0]))
+    f.write("#[macro_export]\n")
+    f.write("macro_rules! atom {\n")
+    f.writelines(['("%s") => { $crate::atom_macro::unsafe_atom_from_static($crate::atom_macro::%s) };\n'
+                 % (atom[1], symbolify(atom[0])) for atom in atoms])
+    f.write("}\n")