DPWA: more instruction on In-Product Help bubble

Add instruction to PWA install In-Product Help bubble about how to
install the app with text "by clicking the install button". This is
important for users who use screenreader, as they can't see the install
button is highlighted.

Bug: 1164050
Change-Id: Ia0a96c13433f385be557dc96c856d60737abdbb7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2616484Reviewed-by: Daniel Murphy <dmurph@chromium.org>
Reviewed-by: Ken Rockot <rockot@google.com>
Commit-Queue: Phillis Tang <phillis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#841279}

chrome/app/generated_resources.grd

@@ -5988,7 +5988,7 @@ Keep your key file in a safe place. You will need it to create new versions of y
         Reopen a tab if you accidentally closed it
       </message>
       <message name="IDS_DESKTOP_PWA_INSTALL_PROMO" desc="Text shown on promotional UI appearing next to the PWA install icon">
-        To get back here quickly, install <ph name="APP_NAME">$1<ex>Google Maps</ex></ph>
+        To get back here quickly, install <ph name="APP_NAME">$1<ex>Google Maps</ex></ph> by clicking the install button
       </message>
       <message name="IDS_REOPEN_TAB_PROMO_SCREENREADER" desc="Text announced encouraging users to reopen a tab with the given shortcut">
         <ph name="SHORTCUT">$1<ex>CTRL+SHIFT+T</ex></ph> can reopen accidentally closed tabs

chrome/app/generated_resources_grd/IDS_DESKTOP_PWA_INSTALL_PROMO.png.sha1

-34a9e96d0617141c5d1e542741566260cb823b75
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+aa0c0ad4cc0e1bf32e30103572a06666c14b01c3
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