Patch 3.3: Refactored cocoa_spelling_engine_mac to be generic.
Mostly just replacing CocoaSpellingEngine with PlatformSpellingEngine,
a class that can be used by any system that wants to use
its own spellchecker.
If the implementation of the PlatformSpellingEngine ends up being
significantly different for Mac and Android, may need to split the
implementation into separate source files in the future.
Part 3 of a 3 part breakdown of
https://codereview.chromium.org/
1204293003.
The breakdown will also include some changes to the original CL.
BUG=415302
Review URL: https://codereview.chromium.org/
1231103004
Cr-Commit-Position: refs/heads/master@{#340206}