feat: Change default network request timeout (shaka-project#3024)

The old default was 0 (effectively infinite, no timeout).  The new default is 30s.

Closes: shaka-project#1578

externs/shaka/net.js

@@ -33,6 +33,7 @@
  *   For example, 0.5 means "between 50% below and 50% above the retry delay."
  * @property {number} timeout
  *   The request timeout, in milliseconds.  Zero means "unlimited".
+ *   <i>Defaults to 30000 milliseconds.</i>
  *
  * @tutorial network-and-buffering-config
  *

lib/net/backoff.js

@@ -139,7 +139,7 @@ shaka.net.Backoff = class {
       baseDelay: 1000,
       backoffFactor: 2,
       fuzzFactor: 0.5,
-      timeout: 0,
+      timeout: 30000,
     };
   }