DevTools: fix image resource view

This patch sets default 'flex' property for resource-view items so that
the won't shrink and overlap.

BUG=352254

Review URL: https://codereview.chromium.org/196343022

git-svn-id: svn://svn.chromium.org/blink/trunk@169416 bbb929c8-8fbe-4397-9dbb-9b2b20218538