Use StringBuilder::appendLiteral() / StringBuilder::append(char) when possible

Use StringBuilder::appendLiteral() / StringBuilder::append(char) when possible
instead of StringBuilder::append(char*), as this is a bit more efficient.

R=adamk@chromium.org, rob.buis@samsung.com

Review URL: https://codereview.chromium.org/482753002

git-svn-id: svn://svn.chromium.org/blink/trunk@180497 bbb929c8-8fbe-4397-9dbb-9b2b20218538