DevTools: Fix transparent retainers view grid.

When retainers view is resized to its maximum in heap allocation
tracking profiler, another view becomes visible behind it.
That is now fixed.

Review URL: https://codereview.chromium.org/143623006

git-svn-id: svn://svn.chromium.org/blink/trunk@166903 bbb929c8-8fbe-4397-9dbb-9b2b20218538