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This is a reland of 9124dffe

Reason for reland: This was breaking Android N by accessing the base
context too early: crbug.com/1156018c. Changes from original are from
patchset 1 on.

Original change's description:
> Preload chrome split on startup
>
> The chrome split can take a long time to load, so we get significant
> improvements in startup time if this work is offloaded to a background
> thread. Ideally, we would give as much time as possible to allow this
> preload to finish, which means running as much code as possible which
> does not depend on the chrome split during the preload. This required
> refactoring much of attachBaseContext into the base module, and moving
> the pieces which could not be easily moved to the base module into
> onCreate.
>
> The order of startup generally goes:
> 1. Application.attachBaseContext
> 2. Each provider is instantiated in order, and onCreate is called
> 3. Application.onCreate
>
> If a provider lives in a split, it will call createContextForSplit on
> the application. This will end up being the first thing needing the
> chrome split, which will wait for the preload to complete. This is why
> ChromeFileProvider was moved to the base module, so there could be more
> time to preload.
>
> Pinpoint runs:
> Pixel 2: https://pinpoint-dot-chromeperf.appspot.com/job/16925180d20000
> -7% messageloop_start_time
> -3% first_contentful_paint_time
> -3.8% navigation_commit_time
>
> Android Go: https://pinpoint-dot-chromeperf.appspot.com/job/115e53e7520000
> -18.7% messageloop_start_time
> -1.5% first_contentful_paint_time
> -2.1% navigation_commit_time
>
> A couple notes:
> - setUsageAndCrashReportingFromNative was removed from UmaUtils because
>   it is never used, and allowed for much more easily bringing UmaUtils
>   into the base module.
> - I wanted to make sure PureJavaExceptionHandler is still set up as
>   early as possible, so left it in attachBaseContext. This required
>   loading PureJavaExceptionReporter by reflection if an exception
>   happens, since it has many hooks into chrome code.
> - MainDexApplicationImpl is no longer needed since all the common code
>   was moved to SplitCompatApplication
>
> Bug: 1150600
> Change-Id: Idd6c11293e47fa5e8bd6eb30c14535e60c204867
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2570125
> Commit-Queue: Clark DuVall <cduvall@chromium.org>
> Reviewed-by: Andrew Grieve <agrieve@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#833923}

Bug: 1150600
Change-Id: Id9de8d25e74b18199fa10fb12a669384908acdb6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2577546
Commit-Queue: Clark DuVall <cduvall@chromium.org>
Reviewed-by: Andrew Grieve <agrieve@chromium.org>
Cr-Commit-Position: refs/heads/master@{#834317}