AMP: check if class exists before to use it.

[jeherve]

Not ready for review yet. Waiting on D26821-code to land

Changes proposed in this Pull Request:

This is important for files that are currently in sync with WordPress.com,
where we don't include the Jetpack_AMP_Support class yet.

See D26821-code for more details.

Testing instructions:

• All features should remain as before when the AMP plugin is active. (see AMP: move checks for AMP requests later, inside modules. #11195 for some examples)
• You should also not see any errors / notices in your logs.

Proposed changelog entry for your changes:

• None

[jetpackbot]

Thank you for the great PR description!

When this PR is ready for review, please apply the [Status] Needs Review label. If you are an a11n, please have someone from your team review the code if possible. The Jetpack team will also review this PR and merge it to be included in the next Jetpack release.

Scheduled Jetpack release: May 7, 2019.
Scheduled code freeze: April 30, 2019

Generated by 🚫 dangerJS against 9e58acb

[jetpackbot]

Thank you for the great PR description!

When this PR is ready for review, please apply the [Status] Needs Review label. If you are an a11n, please have someone from your team review the code if possible. The Jetpack team will also review this PR and merge it to be included in the next Jetpack release.

Scheduled Jetpack release: June 4, 2019.
Scheduled code freeze: May 28, 2019

Generated by 🚫 dangerJS against c5b3a13

[jeherve]

Closing this now, this was done in #12140