Bug 1408537: Slightly increase the epsilon in MathML reftest "font-inflation-1.html", so that it passes on local linux reftest runs. r=jfkthame

The prior epsilon (2px) seems like an arbitrarily chosen value, which must have
been large enough to allow for small sizing differences on our continuous
integration platforms.  But it's not quite large enough to handle sizing
differences on my local linux machine (I'm guessing due to font differences).

Note that this doesn't reduce the sensitivity of the test.  In the test's
anticipated failure mode (when font inflation fails to apply on some element),
the sizing differences are an order of magnitude larger than this amended
epsilon.

MozReview-Commit-ID: Iz7jqRvFh8w

--HG--
extra : rebase_source : 573e42c3237b11d6747ed45bc3ae3d86aeb0a4b3

layout/reftests/mathml/font-inflation-1.html

@@ -14,7 +14,7 @@
     </style>
     <script type="text/javascript">
       function almostEqual(aX, aY) {
-        var epsilon = 2;
+        var epsilon = 2.6;
         return Math.abs(aX - aY) < epsilon;
       }