Inject styles into Shadow DOM when inside Shadow DOM (#5763)

[Simon-Tang]

• Injects the inline <style> used by ChartJS into document.head and each Shadow Root as needed
• Adds a node[EXPANDO]: { containsStyles: boolean } to document.head and each Shadow Root that contains a ChartJS instance
• Removes the flags platform._style and platform._loaded since we now need to keep track of multiple <style> insertions

To-do(?):

• Karma tests to test responsive behaviour inside Web Components
• Update documentation to include the need for manually including the <script> inside Shadow DOMs in the case of disableCSSInjection: true - example workaround

Closes: #5763

[kurkle]

Looks good!

Can you add a test pen where the functionality can be verified? (Fork https://stackblitz.com/edit/polymer310-testcase-chartjs for example)

[Simon-Tang]

Thanks @kurkle. I'm not sure what's up with the error I'm getting when using customElements but here is what I've been using for testing:

https://stackblitz.com/edit/polymer310-testcase-chartjs-zxndkp

I copied my exact testing page into index2.html in the above. I'm getting this error:

Error: Failed to construct 'HTMLElement': Please use the 'new' operator, this DOM object constructor cannot be called as a function.

If any reviewer has the patience to help out with this I would appreciate it a lot! I'm not really sure if I'm missing something obvious to make this work. However I would expect the pen you linked (https://stackblitz.com/edit/polymer310-testcase-chartjs) to also work with the changes in this PR.

[kurkle]

Working test case:
https://stackblitz.com/edit/polymer310-testcase-chartjs-avhczw