Add missing public_deps for //content/public/app:xxx GN targets.
Add missing public_deps for //content/public/app:xxx GN targets.

The non-component build version of the //content/public/app:both target
provides the public_app_shared_sources as sources, whereas the component
build version of the same target does not.

This causes issues for targets outside //content that need to depend on
//content/public/app/content_main.h and other files in
public_app_shared_sources from //content/public/app/BUILD.gn, because
the non-component build has to be special cased.

This CL fixes the public_deps for //content/public/app:xxx for the
component build to make the dependency on //content a public_deps entry.

This includes the targets:
//content/public/app:browser
//content/public/app:child
//content/public/app:both

BUG=597830

Review-Url: https://codereview.chromium.org/1914933004
Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#390103}

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