Add FILE_MISSING in PpapiThread::LoadResult for UMA reporting.

Add FILE_MISSING in PpapiThread::LoadResult for UMA reporting.

Missing plugin file is a common cause of plugin loading failure. Explictly
report this case instead of relying on system error codes.

Note that for plugins with dependencies, this only catches the case that the
real plugin file is missing. Missing dependencies will still be reported through
error codes. One example of this is that the widevinecdmadapter.dll file depends
on widevinecdm.dll.

BUG=345852
Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=276052

[xhwang, 2014-06-06 01:02:24]

PTAL

[ddorwin, 2014-06-06 01:21:48]

lgtm

[xhwang, 2014-06-06 16:05:26]

jar: Please OWNERS review the histogram.xml change.

piman: Please OWNERS review content/ppapi_plugin/*
My understanding is that PathExists() is called before the sandbox is
initialized. So the call should work fine. Please let me know if I miss
anything.

[piman, 2014-06-09 14:38:33]

lgtm

[jar (doing other things), 2014-06-09 18:50:28]

histograms.xml lgtm

[xhwang, 2014-06-09 21:07:13]

The CQ bit was checked by xhwang@chromium.org

[commit-bot: I haz the power, 2014-06-09 21:08:45]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/xhwang@chromium.org/320523002/1

[commit-bot: I haz the power, 2014-06-10 11:14:13]

FYI, CQ is re-trying this CL (attempt #1).
The failing builders are:
  ios_dbg_simulator on tryserver.chromium
(http://build.chromium.org/p/tryserver.chromium/builders/ios_dbg_simulator/bui...)
  win_chromium_rel on tryserver.chromium
(http://build.chromium.org/p/tryserver.chromium/builders/win_chromium_rel/buil...)

[commit-bot: I haz the power, 2014-06-10 16:31:24]

Change committed as 276052