languageSettingsPrivate API definition and stub implementation

languageSettingsPrivate API definition and stub implementation

The IDL was approved by API review and got l-g-t-m from kalman@ at https://codereview.chromium.org/1102693002/

This IDL is the same, modulo stevenjb's comments on that CL and some bug fixes.

BUG=479043

R=stevenjb@chromium.org,kalman@chromium.org,asvitkine@chromium.org
TBR=dbeam@chromium.org

kalman:
  chrome/common/extensions/api/_api_features.json
  chrome/common/extensions/api/language_settings_private.idl (rubberstamped in 1102693002)
  chrome/common/extensions/api/schemas.gypi

asvitkine:
  extensions/browser/extension_function_histogram_value.h
  tools/metrics/histograms/histograms.xml

dbeam:
  (generated file: third_party/closure_compiler/externs/language_settings_private.js)

Review URL: https://codereview.chromium.org/1155093006

Cr-Commit-Position: refs/heads/master@{#338632}