Match TextMetrics emHeightDescent with the rest of the API

"emHeightDescent" was the only descent where "positive" meant "above the baseline". This fixes that and makes it more aligned with the other descents, where "positive" means "below the baseline".

source

@@ -63279,8 +63279,8 @@ v6DVT (also check for '- -' bits in the part above) -->
    <dd><p>The distance from the horizontal line indicated by the <code
    data-x="dom-context-2d-textBaseline">textBaseline</code> attribute to the lowest bottom of the em
    squares in the <span>line box</span>, in <span data-x="'px'">CSS pixels</span>; positive numbers
-   indicating that the given baseline is below the bottom of that em square (so this value will
-   usually be negative). (Zero if the given baseline is the bottom of that em square.)</p></dd>
+   indicating that the given baseline is above the bottom of that em square. (Zero if the given baseline
+   is the bottom of that em square.)</p></dd>
 
    <dt><dfn><code data-x="dom-textmetrics-getBaselines">getBaselines()</code></dfn> method</dt>