Fix inherit from constructor chain

[tlouisse]

Reference Issue

Fixes #467

Instead of checking 'current' constructor level only, this checks the whole constructor chain for static get styles. In this way, it never misses styles defined on constructors never 'ran' before (like mixins): https://stackblitz.com/edit/lit-element-request-update-bug-lspgdq?file=test.js

This could however mean that this._styles is created redundantly for constructor layers that should have the same outcome as their parents(and therefore should be able tor reuse this._styles). A code example that addresses this problem can be found here:
https://github.com/tlouisse/lit-element/blob/fix/getStylesFromConstructorChainAlternative/src/lit-element.ts#L92
But I'm not sure if this is more performant and outweighs the benefits of less code.

Let me know what you think :)

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[googlebot]

CLAs look good, thanks!

[tlouisse]

Closing, since already fixed via #470