Remove faulty copy constructor and assignment operator from WebCore::SegmentedString.

The old implementation was just copying each member one by one except m_currentChar,
but the code for m_currentChar is broken for strings that got two pushes.
(There, m_pushedChar1 must precede m_pushedChar2 according to other methods
like toString(), push(), or append().)

For correct implementation, the compiler-default copy operation is sufficient.
In old days the m_currentChar member was a pointer and required special treatment.

BUG=330808
TEST=blink_platform_unittest --gtest_filter='SegmentedStringTest.*'

Review URL: https://codereview.chromium.org/154003004

git-svn-id: svn://svn.chromium.org/blink/trunk@166463 bbb929c8-8fbe-4397-9dbb-9b2b20218538