CustomEvent serialized detail unit test. This is a re-commit.
The original commit was reverted because it caused a
ListenerLeakTest failure.

Added test for CustomEvent init SerializedScriptValue assert.
This is a simple test of the initCustomEvent internal API to
make sure that it can be called, and that after
initialization, the detail getter can be successfully used.

Review URL: https://chromiumcodereview.appspot.com/19289005

git-svn-id: svn://svn.chromium.org/blink/trunk@154277 bbb929c8-8fbe-4397-9dbb-9b2b20218538