Fix WindowContainerThumbnailTest

A recent commit to WindowContainerThumbnail changed makeSurface
to makeChildSurface but the test was already marked as flaky and
so the CL was able to pass presubmit without updating the test.

Bug: 153708519
Test: WindowContainerThumbnailTest
Change-Id: Ic1ef7cb941b67f2f807d812a6a4cce82bdd917ac
1 file changed