DEPS.xwalk: Roll chromium-crosswalk (4e78cd0 -> 88fc58a).

88fc58a Merge pull request #198 from rakuco/backport-testwebserver-commits
4adebb3 [Backport] Allow forcing TestWebServer to use a specific port number.
0622b0f [Backport] Start server thread in new TestWebServer.start() method rather than constructor

This roll requires changing several Android tests because
TestWebServer's API in Chromium has changed (we would need to do this
anyway when moving to M40). The changes are pretty much the same ones
that were done in Chromium commit c28e68d ("Start server thread in new
TestWebServer.start() method rather than constructor").

BUG=XWALK-2755
[Android] Hardcode the port number the CORS tests use.

In M39, TestWebServer chose a random open port to listen in, but for our
CORS+XHR tests we need it to be listening in a specific port that we
then fetch via XHR.

This fixes org.xwalk.runtime.client.test.CrossOriginXhrTest and
org.xwalk.runtime.client.embedded.test.CrossOriginXhrTest.

BUG=XWALK-2755
Merge pull request #2519 from rakuco/fix-cors-xhr-tests

[Android] Fix failing Cross-Origin XHR tests.