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This is another incremental change towards the goal of having the
spellcheck service initialized on demand, so that there is no
performance hit from loading DLLs supporting the Windows platform
spellchecker on browser startup. The previous change in this effort:

    2239682: Windows Spellcheck: Further work on delay init of
    spellcheck service
    https://chromium-review.googlesource.com/c/chromium/src/+/2239682

listed the following remaining work to be done:

1) In first run scenarios we would still need to load the dictionaries
at browser startup, in order to assure that even if a user never uses
spellcheck in their first browser session after installation,
spellcheck will be enabled for their primary preferred language in the
case where the language has Windows platform but not Hunspell support
(a leading example being Arabic).
2) On going to a page with editable content without first visiting
the languages settings page, clicking in the editable content should
initialize the spellcheck service in the browser process.
3) On right-clicking in editable content to bring up the context menu,
the spellcheck service needs to be instantiated in order to populate
the languages presented in the spellcheck languages submenu.

This CL accomplishes (2). When focus enters editable content, the
method SpellCheckProvider::RequestTextChecking is called from
Blink code in the renderer process. Previously if the spellcheck
service was not initialized, the method would exit without performing
a text check, because there were no native or Hunspell dictionaries
loaded. Now a mojo call is made to asynchronously initialize the
spellcheck service in the browser process. When the dictionaries are
loaded, a callback is made to the renderer, and the text checking
can continue as before.

In all my testing so far this change also fixes (3) as a side effect,
since I haven't discovered any way to bring up the context menu for
editable content without putting the focus there, and gaining focus
invokes a spellcheck request (scenario 2) and thus initializes the
spellcheck service. So unless I am still missing some other scenario
that is broken by delayed initialization, only scenario (1) needs
to be fixed before the WinDelaySpellcheckServiceInit feature flag
can be enabled by default.

Bug: 1064351
Change-Id: I972e76c3052f83f9e05d57086a7cb27e5d9f6ef5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2283370Reviewed-by: Rouslan Solomakhin <rouslan@chromium.org>
Reviewed-by: Ken Buchanan <kenrb@chromium.org>
Reviewed-by: Guillaume Jenkins <gujen@google.com>
Commit-Queue: Bruce Long <brlong@microsoft.com>
Cr-Commit-Position: refs/heads/master@{#787864}