As outlined in the design document linked below, we're removing the
support for the non-standard Function.displayName property for the
purpose of Error.stack and DevTools Inspector stack traces. The
motivation here is that the negative lookup is costly, and we have
Function.name as a standard alternative (configurable since ES6 for
exactly this reason).

I dediced to go with JSFunction::GetDebugName(), since
JSFunction::GetName() was confusing in that it'd only get the "name"
property's value if it's a data property, but not with accessors.
JSFunction::GetDebugName() makes it clear that this is really a debug
helper function and might not give you the "name" property value.

Doc: https://bit.ly/devtools-function-displayName-removal
Bug: v8:8742, chromium:1177685, chromium:1077657, chromium:17356
Change-Id: I7717585cbace626174b2f2ed2a4f68f75429eca1
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2692189
Commit-Queue: Benedikt Meurer <bmeurer@chromium.org>
Reviewed-by: Yang Guo <yangguo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#72715}