ambient: Change one Settings string

Change the label of topic source group in the Settings page to "Photo
Frame".

Bug: none
Test: manual
Change-Id: I3df69244186fde84874bfa3fd828b8a577d24712
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2342304
Auto-Submit: Tao Wu <wutao@chromium.org>
Reviewed-by: Xiaohui Chen <xiaohuic@chromium.org>
Reviewed-by: Kyle Horimoto <khorimoto@chromium.org>
Commit-Queue: Kyle Horimoto <khorimoto@chromium.org>
Cr-Commit-Position: refs/heads/master@{#796008}

chrome/app/os_settings_strings.grdp

@@ -475,7 +475,7 @@
     Off
   </message>
   <message name="IDS_OS_SETTINGS_AMBIENT_MODE_TOPIC_SOURCE_TITLE" desc="Label for the radio button group of ambient mode topic source, where the screen contents come from.">
-    Background
+    Photo Frame
   </message>
   <message name="IDS_OS_SETTINGS_AMBIENT_MODE_TOPIC_SOURCE_GOOGLE_PHOTOS" desc="Label for the radio button to choose the topic source from Google Photos. The radio button allows the user to set Google Photos for ambient mode, which shows photos to the user when their device is idle.">
     Google Photos

chrome/app/os_settings_strings_grdp/IDS_OS_SETTINGS_AMBIENT_MODE_TOPIC_SOURCE_TITLE.png.sha1

-c8a3be00b8006564c838bae42958dcef728e44c7
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+6c9a91e5dcdcdc3cc1ea1b1fe9b5a1f5f76f9179
\ No newline at end of file