Update Ahem capitalization in LayoutTests

The way tests fonts are side loaded when running layout tests on Windows
with DirectWrite requires the name to be specified with the correct case
as matching is case sensitive. This is different from how normal system
fonts are handled but as this facility is only used for layout tests the
extra overhead of implementing case insensitive matching doesn't seem
worth it.

This change updated all layout tests using the Ahem font as a test
provided system font to request it using "Ahem".

BUG=333029
R=dglazkov@chromium.org, cpu@chromium.org

Review URL: https://codereview.chromium.org/291853004

git-svn-id: svn://svn.chromium.org/blink/trunk@174325 bbb929c8-8fbe-4397-9dbb-9b2b20218538