content: Fix for GN Mac build.
content: Fix for GN Mac build.

Fix the following error:

ninja: error: '../../content/shell/shell_content_main.cc', needed by 'obj/content/shell/libcontent_shell_framework/shell_content_main.o', missing and no known rule to make it

Tested on Mac with the following command lines:

$ gn gen out-gn/Release --args='is_debug=false is_component_build=false symbol_level=1'
$ ninja -C out-gn/Release

The fix is simple as shell_content_main.cc lives in content/shell/app rather than
content/shell.

BUG=431177
TBR=brettw@chromium.org

Review URL: https://codereview.chromium.org/1423883002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#355993}

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