[autotest] Change ChromeTestBase to work with pre-installed chrome test binaries.

Instead of pulling chrome test binaries as a dep (which takes too long on VMs),
we'll be pre-installing them on test images.  Update the ChromeTestBase class
to expect this new behavior.

BUG=8544
TEST=Build a test image with chrome > -r35, run desktopui_BrowserTest

Change-Id: I013b689b4ad7fe5e47760312da573a9a2b29d669

Review URL: http://codereview.chromium.org/4557002
1 file changed