Fixes for re-enabling more MSVC level 4 warnings: chrome/ edition

Fixes for re-enabling more MSVC level 4 warnings: chrome/ edition

This contains fixes for the following sorts of issues:
* Signedness mismatch
* Assignment inside conditional
* Constant value was truncated
* Possibly-uninitialized local variable

BUG=81439
TEST=none

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=283305

[Peter Kasting, 2014-07-07 20:55:47]

https://codereview.chromium.org/377683002/diff/1/chrome/utility/media_galleri...
File chrome/utility/media_galleries/pmp_column_reader_unittest.cc (right):

https://codereview.chromium.org/377683002/diff/1/chrome/utility/media_galleri...
chrome/utility/media_galleries/pmp_column_reader_unittest.cc:143:
bad_magic_bytes[0] = static_cast<char>(-128);
0xff is considered by the compiler to be a positive value, and thus is larger
than what a signed char can hold, triggering a "constant truncated" warning.

To me, this suggests that this code should be using uint8_t, or at least
unsigned char, instead.  However, making that change turns out to be fairly
far-ranging (we have various routines in Chrome that use "vector<char>" to mean
"string of bytes"), so eventually I gave up and just made the change here.

[Peter Kasting, 2014-07-07 21:19:02]

Somehow I mailed this without setting a reviewer first...

[Peter Kasting, 2014-07-11 17:21:46]

Friendly ping

[Peter Kasting, 2014-07-14 22:16:52]

Scott, no good deed goes unpunished, so I'm going to send you another two of
these, since Darin seems to be unresponsive as well.  Here's 1 of 2.

[sky, 2014-07-15 04:16:25]

Are you telling me I should instead be slower in my reviews?

LGTM

[Peter Kasting, 2014-07-15 18:55:00]

The CQ bit was checked by pkasting@chromium.org

[commit-bot: I haz the power, 2014-07-15 18:57:29]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/pkasting@chromium.org/377683002/1

[commit-bot: I haz the power, 2014-07-16 00:34:56]

Change committed as 283305