DevTools: Initialize private member _anchorElement to null in constructor
DevTools: Initialize private member _anchorElement to null in constructor

The private class member _anchorElement was previously not Initialized in the
constructor, besides improving the code style this also changes the
_anchorElement from an inferred type to a declared type in the closure
compiler typing system which improves type checking [1].

[1]: https://github.com/google/closure-compiler/wiki/Types-in-the-Closure-Type-System#differences-between-inferred-and-declared-types

Test: Click the source link on a console message and ensure the source is revealed.
Change-Id: I17587d9504128c0c08926c4521a0106e584508ef
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1816963
Reviewed-by: Yang Guo <1cf0af8245a3a17a632a2c73531392b0f9761c76@chromium.org>
Commit-Queue: John Emau <c153e6e600be23fb28cec0b08551440e4bf7575d@microsoft.com>
Cr-Original-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#702936}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: d79256fc6f4350eef567b8887928fb2242257803

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