use - and = to zoom text in slides

BUG=skia:
TBR=
NOTRY=True

Review URL: https://codereview.chromium.org/668023002
diff --git a/resources/slides.lua b/resources/slides.lua
index c74f47a..419e812 100644
--- a/resources/slides.lua
+++ b/resources/slides.lua
@@ -412,8 +412,8 @@
     p = prev_slide,
     r = spawn_rotate_animation,
     s = spawn_scale_animation,
-    u = function () scale_text_delta(gTemplate, 1) end,
-    d = function () scale_text_delta(gTemplate, -1) end,
+    ["="] = function () scale_text_delta(gTemplate, 1) end,
+    ["-"] = function () scale_text_delta(gTemplate, -1) end,
 }
 
 function onCharHandler(uni)