My personal feeling is that this would be possibly used by a tiny minority of users and doesn't work well with the tab interaction that requires unobstructed access to the top of the screen.
This could possibly be made to work with some resistance along the top of the workspace / bottom of the top-aligned shelf border but that might be complicating it for the users.

Passing it along though.

Has more to do with MD consistency: while Chrome OS currently has its Shelf on the bottom, Android has (and always has had) its equivalent (namely, the status bar — the core navigation bar at the bottom is only applicable on the mobile end) on the top, resulting in a massive change in UI layout from mobile to desktop, in direct violation of one of the MD guidelines (namely, seamless scalability) specified at I/O two years ago. Also, unrestricted access to the top of the screen may be a problem for browsing, but not packaged and/or ARC app use — the wealth of Play Store apps set to show up on Chrome OS within the next month or two would not be affected by Shelf position as much as the browser currently is. I do agree that it's a valid argument, however.

Supporting left/right docking today is used by a very small segment of users for the complications provided. We have no plans to support moving the shelf to the top of the screen.

Alternative feature request:  Issue 625523