The constructor that accepts a size for vector default initializes the
elements. Using reserve() instead avoids this but still ensures the
capacity.

Change-Id: I8effb62ba1f9598a0081c2c959b2f7a5d6c587e6
Reviewed-on: https://chromium-review.googlesource.com/1186407Reviewed-by: Marja Hölttä <marja@chromium.org>
Commit-Queue: Peter Marshall <petermarshall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#55348}