Prevent dom-repeat's forwardHostProp from retaining HTMLElement `this`

[sigurdschneider]

TL;DR: The function __ensureTemplatized in elements/dom-repeat.js caused the templatizeInstanceClass to retain a reference to the HTMLElement (i.e. the DomRepeat object) that triggered creation of the templatizeInstanceClass (see templatize.js:566).

The function ensureTemplatized in elements/dom-repeat.js contained an arrow function definition for the MutationObserver and the function for forwardHostProp in it; the arrow function definition forces context allocation of this so it can accesses it later. This introduces false sharing with forwardHostProp: that function refers to the same context, and transitively also to the this of `ensureTemplatized, although it neither references nor accesses it. Note thatforwardHostPropwill get it's ownthis` variable on each invocation). This results in the HTMLElement that originally created the __ctor to be retained (i.e. not cleaned up by the GC). The retainment path can be seen in the following heap snapshot: https://imgur.com/a/javUpRx

In the screenshot, the tab on the top shows the detached HTML element (the HTMLElement that created the ctor, which I suspect is really a DomRepeat object inheriting from HTMLElement) and the bottom shows the retainment chain. The retainment chain is to be read as follows: Line 1: this is a property of a context. Line 2: that context is the context of the function forwardHostProp. Line 3: That function is stored in a property called forwardHostProp of an Object. Line 4: that Object is stored in a property `templatizeOptionsin an object with constructorTemplateLegacy. Line 5: ThatTemplateLegacyobject is theprotoof an object constructed with constructorklass. Line 6: That object, in turn, is theprotoof an object constructed withklass$jscomp$1. Line 7: that object is a property called__templatizeInstance` on am HTMLElement object. Line 8: that HTMLElement is an internal property of a ShadowRoot object, and so on.

This CL fixes this problem by moving the arrow function into its own method __waitForTemplate. This causes the context of __waitForTemplate to get a context-allocated this, but prevents the context of __ensureTemplatized from having a context-allocated this. Unfortunately, it is hard to verify this fix, because inspecting the function object for forwardHostProp shows all these context collapsed into 'Local'. Basically, the only way to verify a fix for an issue like this atm. is to get a reproducible way to produce a heap snapshot that exhibits the problem, and then verifying that the problem is not present anymore after applying the fix. In my repro (navigating between to pages of Gerrit code review tool) this got rid of about 37 detached HTMLElements.

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