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With the configuration of software fallback, it is getting harder to
implement unit test to test the case of TPM being disabled. Thus, this
CL convert the software fallback to be inject-able during unittest.

As for naming, since "enabled" doesn't explicitly express when we use
it, so the flag is changed to "allowed" so it is more clear about its
intention.

BUG=b:172748724
TEST=unit_tests.

Change-Id: I1aff3a36d1bc4524438a275b92d09ec859e86ba8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2581706
Commit-Queue: Leo Lai <cylai@google.com>
Reviewed-by: Achuith Bhandarkar <achuith@chromium.org>
Reviewed-by: Maksim Ivanov <emaxx@chromium.org>
Cr-Commit-Position: refs/heads/master@{#836487}