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The interactive tutorial uses hard-coded strings to specify text
content. Included in this patch:

1. Add tutorial_common.js, which can be used to share code between
the tutorial and lesson components
2. Add a function, getMsg(), to fetch localized strings
3. Change some hard coded strings to message IDs
4. Clean up BUILD.gn

fr and es) and verify that content gets translated.

Bug: 1075752
Test: Manually load the tutorial in several locales (I used
Change-Id: I5784f1466f3646e31a41145478db9dfcb766f70e
AX-Relnotes: N/A
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2254345
Commit-Queue: Akihiro Ota <akihiroota@chromium.org>
Reviewed-by: Dominic Mazzoni <dmazzoni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#813361}