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This patch changes the logic the list of zoom values are computed for a
given display. The new logic has 2 scenarios for listing the zoom values
for a given display.

1) Displays with device scale factors assigned to them, will now have
   zoom values ranging from the inverse of device scale factor to
   device scale factor. If there are still slider ticks avaiable we use
   them to add zoom levels beyond the device scale factor.
   Doing this allows the user to go to the native resolution of the
   display and on the other hand it also allows them to zoom in if
   required. We no longer allow the user to go to a zoom below the native
   resolution of the display as this is a very unlikely scenario and
   introduces artifacts. This also gives finer control to the user in
   setting the zoom level.
   How this effects a pixelbook for example?
   On pixelbook we used to have a zoom range of 50% to 175%. Due to the
   wide range, the consecutive values were too far apart and users wanted
   zoom values that were mostly in the range of 70% to 100%.
   With the new change, the range will go from 50% to 130%.

2) Displays with no device scale factors assigned to them will use a
   static list of initialized zoom values.

Bug: 845634
Change-Id: I69a761856dab4e5b37b85420f6f6dfebdb2dead5
Component: Display zoom, display util
Reviewed-on: https://chromium-review.googlesource.com/1069561
Commit-Queue: Malay Keshav <malaykeshav@chromium.org>
Reviewed-by: Ahmed Fakhry <afakhry@chromium.org>
Cr-Commit-Position: refs/heads/master@{#563050}