media/blink is a separate module from media/, and needs it's own export macro.

Currently everything works, but mostly because of happy accidents. When adding new code and tests to media/blink I discovered that some symbols were not available on component builds because they were imported rather than exported. This fixes that. Unfortunately I do not remember which symbols I had a problem with.

Review URL: https://codereview.chromium.org/1430063003

Cr-Commit-Position: refs/heads/master@{#358485}