Adjust test thresholds for OSX 10.11

Slightly increase some thresholds for tests that were failing on OSX
10.11.

TBR=hongchan@chromium.org
BUG=626449
TEST=constructor/periodicwave.html

Review-Url: https://codereview.chromium.org/2345913004
Cr-Commit-Position: refs/heads/master@{#419307}

third_party/WebKit/LayoutTests/webaudio/constructor/periodicwave.html

@@ -116,7 +116,7 @@
             }
             return expected;
           },
-          2.7106e-5).then(taskDone);
+          2.7143e-5).then(taskDone);
       });
 
       audit.defineTask("2: real periodicwave test", function (taskDone) {
@@ -131,7 +131,7 @@
             }
             return expected;
           },
-          2.7106e-5).then(taskDone);
+          2.7143e-5).then(taskDone);
       }),
 
       audit.defineTask("3: real periodicwave test", function (taskDone) {
@@ -147,7 +147,7 @@
           }
           return expected;
         },
-        5.4211e-5).then(taskDone);
+        5.4285e-5).then(taskDone);
       });
 
       audit.defineTask("1: imag periodicwave test", function (taskDone) {