Don't allow direct const char* trace value

Remove TraceStringWrapper::operator const char*()

Remove the operator 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 no errornous usages in chromium, but found one error in WebKit.

R=dsinclair
BUG=382789
(perhaps the reason)

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=278061

[dsinclair, 2014-06-17 22:26:21]

lgtm

[Xianzhu, 2014-06-17 23:02:47]

The CQ bit was checked by wangxianzhu@chromium.org

[commit-bot: I haz the power, 2014-06-17 23:06:54]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/wangxianzhu@chromium.org/338653005/40001

[Xianzhu, 2014-06-17 23:23:57]

The CQ bit was checked by wangxianzhu@chromium.org

[commit-bot: I haz the power, 2014-06-17 23:24:38]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/wangxianzhu@chromium.org/338653005/60001

[Xianzhu, 2014-06-18 00:10:01]

Unfortunately, clang thinks "const T*" and "const char(&)[N]" are ambiguous in
overloaded functions. Haven't found a solution.

Will just submit a subset of the change (removing const char* operator of
TraceStringWithCopy).

[Xianzhu, 2014-06-18 00:28:58]

The CQ bit was checked by wangxianzhu@chromium.org

[commit-bot: I haz the power, 2014-06-18 00:30:58]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/wangxianzhu@chromium.org/338653005/80001

[commit-bot: I haz the power, 2014-06-18 14:43:53]

Change committed as 278061