Lazy-build the placeholderTemplate

This script runs on every page, and most pages don't
have 3D images on them, so avoid expensive DOM building.

Bug: T204621
Change-Id: I9e718957a497d24925ba69b82bfcbb1626448c2b
(cherry picked from commit 0e514715cd812a9d39f4e4b3141e4d18b28fcdf5)