Clarify the console error when a PNG icon is not available for add to homescreen.

BUG=761682

Change-Id: I6eff6704ad19ddf9d3aa5b2167af12d9e643b044
Reviewed-on: https://chromium-review.googlesource.com/745684Reviewed-by: Ben Wells <benwells@chromium.org>
Commit-Queue: Dominick Ng <dominickn@chromium.org>
Cr-Commit-Position: refs/heads/master@{#512731}

chrome/browser/installable/installable_logging.cc

@@ -48,7 +48,7 @@ static const char kNoMatchingServiceWorkerMessage[] =
     "check that the service worker for the current page also controls the "
     "start URL from the manifest";
 static const char kNoAcceptableIconMessage[] =
-    "a %dpx square icon is required, but no supplied icon meets this "
+    "a %dpx square PNG icon is required, but no supplied icon meets this "
     "requirement";
 static const char kCannotDownloadIconMessage[] =
     "could not download a required icon from the manifest";