Add option for url_fetch_test to wait for a JS expression

Add option for url_fetch_test to wait for a JS expression

This option extends url_fetch_test to support waiting for a JavaScript
expression to evaluate to true. This is used in the Chrome OS
desktopui_GmailLatency test. The browser goes to the test URL and then runs
the test to completion.

BUG=chromium-os:10869
TEST=test on my workstation on Ubuntu, x86-mario, qemu using a web page which
sets a JS value after a period.

Original Review URL: http://codereview.chromium.org/6249003/

Signed-off-by: Simon Glass <sjg@chromium.org>;
Reviewed-by: Anton Sargent <asargent@chromium.org>;
Reviewed-by: William Chan <willchan@chromium.org>;
Reviewed-by: Thiago Farina <tfarina@chromium.org>;

Committed: http://src.chromium.org/viewvc/chrome?view=rev&revision=71408

[tfarina, 2011-01-13 22:55:45]

FYI!