Do not clobber dynamic public key pins when handling the Strict-Transport-Security header.
Do not clobber dynamic public key pins when handling the Strict-Transport-Security header.

Rather than unilaterally calling EnableHost, do like
ProcessPublicKeyPinsHeader and first look up any existing state for the
given host.

BUG=109691
TEST=Set a pin for a domain that serves HSTS, visit that domain, and then make sure the pin is still present

Review URL: http://codereview.chromium.org/9231029

git-svn-id: de016e52bd170d2d4f2344f9bf92d50478b649e0@118126 0039d316-1c4b-4281-b951-d872f2087c98

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