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This CL adds DevUiModuleProvider classes (Java and C++) as backend to
install and load the DevUI DFM. The C++ API must be called from the UI
thread. It consists of query helpers:
* DevUiModuleProvider::GetInstance().ModuleInstalled(),
* DevUiModuleProvider::GetInstance().ModuleLoaded(),
and asynchronous (i.e., takes callbacks) actions:
* DevUiModuleProvider::GetInstance().InstallModule(),
* DevUiModuleProvider::GetInstance().LoadModule().

The Java backend handles DFM installation, and implements standard
on-demand DFM installation flow. For simplicity, no Java UI elements
(e.g., using Toast or InfoBar) as used. Instead, an interstitial page
that calls the C++ API will be used.

Some sources of complexity, also encountered by VrModuleProvider (upon
which the code is based) are:
* Interaction between C++ and Java DevUiModuleProvider via JNI calls
  and interaction between the singletons.
* Queue management for InstallModule() and LoadModule(), to handle the
  corner case of multiple concurrent install / load requests.

Bug: 927131
Change-Id: I4a6f3e4209857542a63e49ff79871a50bc5da4ab
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1769170
Commit-Queue: Samuel Huang <huangs@chromium.org>
Reviewed-by: Nico Weber <thakis@chromium.org>
Reviewed-by: Andrew Grieve <agrieve@chromium.org>
Cr-Commit-Position: refs/heads/master@{#692087}