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This CL moves a bit of translate logic from chrome/ to the translate
component.

Some IPC messages are still forwarded to chrome (through
ContentTranslateDriver::Observer) because of translate notifications.
This is temporary though, because once translate notifications are
removed, ChromeTranslateClient will no longer need to be an Observer.

Another motivation for this change is that the renderer-side of translate
IPC (TranslateHelper) is moving to the component (in another CL),
and thus the browser-side has to move too for consistency (this CL).

BUG=335087
TBR=msw

Review URL: https://codereview.chromium.org/564743002

Cr-Commit-Position: refs/heads/master@{#295073}