Use the return value to signal success for VTTParser helpers

Convert functions on the form:

ValueType f(..., bool& success);

to the form:

bool f(..., ValueType& value);

Since the latter form tend to integrate more naturally in a
success/failure parsing scheme.
Make sure that the output value is not touched unless parsing was
successful.
Perform some (minor) simplifications due to this change.

Also simplify the check for a trailing '%' in parseFloatPercentageValue.

BUG=305317

Review URL: https://codereview.chromium.org/104713002

git-svn-id: svn://svn.chromium.org/blink/trunk@163248 bbb929c8-8fbe-4397-9dbb-9b2b20218538