Suppress the Bluetooth prompt when Chrome is launched with BT disabled

iOS prompts the user to enable Bluetooth when the CBCentralManager object
is created. Setting the ShowPowerAlert option to NO suppresses the prompt.
This allows Chrome to check the Bluetooth enabled state without prompting
the user.

BUG=630769

Review-Url: https://codereview.chromium.org/2443733002
Cr-Commit-Position: refs/heads/master@{#427083}

ios/chrome/common/physical_web/physical_web_scanner.mm

@@ -109,7 +109,10 @@ enum BeaconType {
     pendingRequests_.reset([[NSMutableArray alloc] init]);
     centralManager_.reset([[CBCentralManager alloc]
         initWithDelegate:self
-                   queue:dispatch_get_main_queue()]);
+                   queue:dispatch_get_main_queue()
+                 options:@{
+                   CBCentralManagerOptionShowPowerAlertKey : @NO
+                 }]);
     unresolvedDevices_.reset([[NSMutableArray alloc] init]);
     [[NSHTTPCookieStorage sharedHTTPCookieStorage]
         setCookieAcceptPolicy:NSHTTPCookieAcceptPolicyNever];