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This CL avoids expensive and repeating work of filling ModuleCache
with modules in NativeUnwinderAndroid::AddInitialModules.
Instead, ModuleCache::RegisterAuxiliaryModuleProvider is used
to register a provider that lazily creates non-elf modules when
needed.

To make that possible, this CL makes explicit new constraints on
Unwinder: a single ModuleCache is "associated" with a Unwinder for its
lifetime and ModuleCache must be outlive by any Unwinders it's
associated with.
A follow up CL will refactor the Unwinder interface to better fit these
constraints.

Change-Id: I05374ed8989061c81cde0ba09605b54e6b2309b9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2514743
Commit-Queue: Etienne Pierre-Doray <etiennep@chromium.org>
Reviewed-by: Mike Wittman <wittman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#842185}