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This CL also includes a few fixes for loading a WebUILoginView instance by itself without associated helper classes also being initialized.

There are three child CLs associated with this one:
- [1] contains fixes on the HTML side.
- [2] makes the lock screen show up correctly in the task manager.
- [3] preloads and reuses the WebUIScreenLocker instance (derived from WebUILoginView).

1: https://codereview.chromium.org/2555453003/
2: https://codereview.chromium.org/2550263002/
3: https://codereview.chromium.org/2553863002/

There is additional context at go/cros-lock-screen-optimization.

BUG=669638
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:closure_compilation

Review-Url: https://codereview.chromium.org/2512473004
Cr-Commit-Position: refs/heads/master@{#437942}