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Add spellcheck functionality to webview. Spellchecking
feature will be enabled if the kEnableAndroidSpellChecker
flag is present.

Historical note:
Blink used to have unified_textchecker_enabled setting
which was false by default in android webview and
needed to be enabled in order for spellchecking to work.
It was recently removed in
https://codereview.chromium.org/2235643002/ though.

BUG=583616, 629609

Review-Url: https://codereview.chromium.org/2264633002
Cr-Commit-Position: refs/heads/master@{#414331}