test: Fix image comparison threshold

Screenshot-based layout tests use the Jimp module to compare images.
But the threshold parameter was set too low (0), causing failures due
to tiny color changes that can't be seen by a human.  The most recent
Chrome update on Windows (86) caused a test failure because some
rendering changes on the edges of an image caused a few pixel values
to change very very slightly.  This fixes the issue by increasing the
threshold from 0 to 0.05.

Change-Id: Ia202c147d30f93779b0b0fed281ee1d256dee371

karma.conf.js

@@ -584,7 +584,12 @@ function WebDriverScreenshotMiddlewareFactory(launcher) {
 
     // Compare the new screenshot to the old one and produce a diff image.
     // Initially, the image data will be raw pixels, 4 bytes per pixel.
-    const diff = Jimp.diff(oldScreenshot, newScreenshot, /* threshold= */ 0);
+    // The threshold parameter affects the sensitivity of individual pixel
+    // comparisons.  Setting it too low means small rendering changes in the
+    // browser can cause failures even when a human can't see the difference,
+    // and setting it too high means human-noticeable changes could go
+    // undetected by a test.
+    const diff = Jimp.diff(oldScreenshot, newScreenshot, /* threshold= */ 0.05);
 
     // Write the diff to disk.  This is used to review when there are changes.
     fs.writeFileSync(