fix(config): wait 20s for browser activity. (#3087)

On low-horsepower CI systems karma frequently times out waiting for phantomjs. We see this
in our Travis-CI runs. Users also see it: 
karma-runner/karma-phantomjs-launcher#126.

docs/config/01-configuration-file.md

@@ -156,7 +156,7 @@ a flaky network link between the Karma server and the browsers.
 ## browserNoActivityTimeout
 **Type:** Number
 
-**Default:** `10000`
+**Default:** `20000`
 
 **Description:** How long will Karma wait for a message from a browser before disconnecting from it (in ms).
 
@@ -581,7 +581,7 @@ Note: Using `'https:'` requires you to specify `httpsServerOptions`.
 
 **Description:** Module used for Karma webserver.
 
-Uses the provided module instead of node's built in `http` or `https` module. The module loaded here must exactly match the interface of node's http module. This can be useful for loading in a module like `node-http2` to allow for http2 support. 
+Uses the provided module instead of node's built in `http` or `https` module. The module loaded here must exactly match the interface of node's http module. This can be useful for loading in a module like `node-http2` to allow for http2 support.
 
 Note: if you're using this to enable `http2` you must also set the `protocol` to `https:` and specify certificates as http2 can only run of https.
 

lib/config.js

@@ -343,7 +343,7 @@ class Config {
     }
     this.browserDisconnectTimeout = 2000
     this.browserDisconnectTolerance = 0
-    this.browserNoActivityTimeout = 10000
+    this.browserNoActivityTimeout = 20000
     this.processKillTimeout = 2000
     this.concurrency = Infinity
     this.failOnEmptyTestSuite = true