WebUI: Enable no-var and prefer-const ESLint rules for most folders.

Specifically:
 - Update chrome/browser/resources/.eslintrc.js to have the desired
   ESLint checks.
 - Explicitly override the default checks for folders that still have
   plenty of violations (bluetooth_internals/, chromeos/, cryptotoken/,
   local_ntp/, media_router/)

Bug: 792774,720034
Change-Id: I505b9ce7773bb706c47757447260630a3c4a38b1
Reviewed-on: https://chromium-review.googlesource.com/c/1357531
Reviewed-by: Dan Beam <dbeam@chromium.org>
Commit-Queue: Demetrios Papadopoulos <dpapad@chromium.org>
Cr-Commit-Position: refs/heads/master@{#615370}
22 files changed