Extension docs: Corrent path to NM user-level directory on Linux

TBR=kalman@chromium.org

Review URL: https://codereview.chromium.org/184713005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@254676 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 254676
Define <webview>.contextMenus.onClicked event for Context Menus API.

This CL exposes .contextMenus.onClicked event per webview which can
independently register for listeners. The tricky part is that
behind the scenes, there is only one Event that fires from the
browser.

BUG=140315
Test=No visible or behavioral change.

Review URL: https://codereview.chromium.org/184993009

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@254677 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 254677