Make content/gpu and content/plugin pass GN check. Significant work for most of content/test
Makes most of content/shell and content/test pass check. These are not all enabled yet pending a few remaining issues that are more difficult to solve.
The private content targets now allow content/test to include headers from them for non-component builds. Checking is disabled for content/test in component builds as described in the comment at the top of //content/BUILD.gn
Renames the ui/events/ipc according to GN style.
Annotates all content and gpu visibility declarations with why the're there, and fixes some incorrect ones that people added without understanding.
Reland of https://codereview.chromium.org/1828483002/ with significant changes.
TBR=jschuh (non-security-sensitive ipc build changes)
Review URL: https://codereview.chromium.org/1833193002
Cr-Commit-Position: refs/heads/master@{#383358}
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