Loosen text measurement expectation.

Cherry-picked out of f0974bc.
Ref #1776 (review)

Author: zbjornson

test/canvas.test.js

@@ -934,8 +934,9 @@ describe('Canvas', function () {
 
       ctx.textBaseline = "alphabetic"
       var metrics = ctx.measureText("Alphabet")
-      // Zero if the given baseline is the alphabetic baseline
-      assert.equal(metrics.alphabeticBaseline, 0)
+      // Actual value depends on font library version. Have observed values
+      // between 0 and 0.769.
+      assert.ok(metrics.alphabeticBaseline >= 0 && metrics.alphabeticBaseline <= 1);
       // Positive = going up from the baseline
       assert.ok(metrics.actualBoundingBoxAscent > 0)
       // Positive = going down from the baseline