DevTools: fixed UI glitches of inlined DataGrid.

1) fix inlined DataGrid columns widths
2) fix left border
3) do not show corner column

BUG=

Review URL: https://codereview.chromium.org/334183003

git-svn-id: svn://svn.chromium.org/blink/trunk@176305 bbb929c8-8fbe-4397-9dbb-9b2b20218538