Remove HeapSnapshotView._mouseDownInContentsGrid

The method was added to switch between value and percent view (https://bugs.webkit.org/show_bug.cgi?id=66988). Now that we always show percents next to the absolute value there is no need in the handler.

BUG=None
R=loislo@chromium.org

Review URL: https://codereview.chromium.org/200123002

git-svn-id: svn://svn.chromium.org/blink/trunk@169228 bbb929c8-8fbe-4397-9dbb-9b2b20218538