exp/sensor: don't allow delays smaller than 10ms on ios

"You can set the reporting interval to be as small as 10 milliseconds
(ms), which corresponds to a 100 Hz update rate, but most app operate
sufficiently with a larger interval."

from https://developer.apple.com/library/ios/documentation/EventHandling/Conceptual/EventHandlingiPhoneOS/motion_event_basics/motion_event_basics.html

Larger delay values makes the library poll more frequently than needed.

Change-Id: Ie3057813209746bff00b8ef534c9ae239396366e
Reviewed-on: https://go-review.googlesource.com/14252
Reviewed-by: David Crawshaw <crawshaw@golang.org>

exp/sensor/darwin_armx.go

@@ -41,13 +41,28 @@ func (m *manager) initialize() {
 	C.GoIOS_createManager()
 }
 
+// minDelay is the minimum delay allowed.
+//
+// From Event Handling Guide for iOS:
+//
+// "You can set the reporting interval to be as small as 10
+// milliseconds (ms), which corresponds to a 100 Hz update rate,
+// but most app operate sufficiently with a larger interval."
+//
+// There is no need to poll more frequently than once every 10ms.
+//
+// https://developer.apple.com/library/ios/documentation/EventHandling/Conceptual/EventHandlingiPhoneOS/motion_event_basics/motion_event_basics.html
+
+const minDelay = 10 * time.Millisecond
+
 func (m *manager) enable(s Sender, t Type, delay time.Duration) error {
-	// TODO(jbd): If delay is smaller than 10 milliseconds, set it to
-	// 10 milliseconds. It is highest frequency iOS SDK suppports and
-	// we don't want to have time.Tick durations smaller than this value.
 	channels.Lock()
 	defer channels.Unlock()
 
+	if delay < minDelay {
+		delay = minDelay
+	}
+
 	switch t {
 	case Accelerometer:
 		if channels.acceleroDone != nil {