Remove TraceStringWithCopy::operator const char*()

Remove it to avoid misuse like the following:
  TRACE_EVENT1("a", "b", "c",
     condition ? TRACE_STR_COPY(short_live_string) : "literal");
In the above case, the argument value is actually const char*, not
TraceValueWithCopy because it is automatically converted back to
const char* with the const char* operator.

Found and fixed one error in WebKit.

Also changed some TRACE_STR_COPY(cstring.ascii().data()) to
cstring.ascii() because we always copy CStrings.

R=dsinclair,haraken
TBR=... changes of callers about TRACE_STR_NO_COPY
BUG=382789 (perhaps the reason)

Review URL: https://codereview.chromium.org/342513003

git-svn-id: svn://svn.chromium.org/blink/trunk@176375 bbb929c8-8fbe-4397-9dbb-9b2b20218538