[autotest]  Display only tests supported by build

The 'Create Job' tab was updated so that an 'Update Tests'
button was placed next to the entry for the image URL.

With this change, pressing the 'Update Tests' button when
a valid build is specified updates the test selection to
only contain tests that are specified in that image.

BUG=chromium:486221
DEPLOY=afe
TEST=Tested changes on a panther moblab.

Change-Id: I9817c3939c9454e7e2b558fa525496cc201fa03a
Reviewed-on: https://chromium-review.googlesource.com/272285
Reviewed-by: Dan Shi <dshi@chromium.org>
Tested-by: Matthew Sartori <msartori@chromium.org>
Reviewed-by: Simran Basi <sbasi@chromium.org>
Commit-Queue: Aviv Keshet <akeshet@chromium.org>
5 files changed