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According to the gn reference (and from brettw@): the "loadable_module"
target type allows you to create an object file that is (and can only
be) loaded and unloaded at runtime, which is exactly what we want for
Clear Key CDM.

Bug: 584449
Test: Existing tests pass.
Change-Id: I2a3a322cb13a7c052c7c43fdd1235ccadd0742ce
Reviewed-on: https://chromium-review.googlesource.com/988814Reviewed-by: Robert Sesek <rsesek@chromium.org>
Reviewed-by: John Rummell <jrummell@chromium.org>
Reviewed-by: Lei Zhang <thestig@chromium.org>
Commit-Queue: Xiaohan Wang <xhwang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#547591}