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This CL changes the Template Expressions from using
a ${ to identify the start of an expression, to using
$i18n{.  The intention is to reduce (eliminate) false
positives when searching for expressions that need
replacement.  This CL also prepares for the replacement
work to be done in a background thread and used on a
broader set of files.

BUG=506009

Review URL: https://codereview.chromium.org/1585923011

Cr-Commit-Position: refs/heads/master@{#371537}