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Introduce a build flag for the remote spelling service, and stop
compiling/instantiating SpellingServiceClient on platforms that
don't use it (Android). Also try to reduce the number of platform
ifdefs in spellcheck code, in favor of buildflag usage. Reduce
the amount of code that's compiled but never used by removing
several SpellCheckHostImpl methods on mac/win.

Also remove a couple of obsolete includes.

Change-Id: I5697a5d3f891f69159d5bba855ee72e11a3ca8be
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1927412Reviewed-by: Rouslan Solomakhin <rouslan@chromium.org>
Reviewed-by: Bo <boliu@chromium.org>
Commit-Queue: Bo <boliu@chromium.org>
Auto-Submit: Evan Stade <estade@chromium.org>
Cr-Commit-Position: refs/heads/master@{#718338}