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This is a part of refactoring to replace all use of gfx::Font with gfx::FontList.  Also this replacement solves a possible vertical alignment issue of text rendering.

See "Background & Problem" section in the following doc why this refactoring is needed.
https://docs.google.com/a/chromium.org/document/d/1D_25fp9B8b9aZJORfAjDIFq61NWvUquZ5xmKH-VcC4k/edit#heading=h.olbd4u4vk3h6

NOTE:
a) Existing Font::GetStringWidth() is now obsolete and should be replaced by gfx::GetStringWidth() in ui/gfx/text_utils.h .
b) 'const' has internal linkage so it doesn't require anonymous namespace (in ui/views/widget/tooltip_manager.cc).

BUG=265485
TEST=Run unittests.

Review URL: https://chromiumcodereview.appspot.com/23483015

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@220509 0039d316-1c4b-4281-b951-d872f2087c98