Fix 'Left shift cannot be represented in int32_t'.
Fix 'Left shift cannot be represented in int32_t'.

In the legacy C part of AGC, an audio level 'cur_level' is represented as

  (1+frac) * 2^(31 - zeros)

The 'zeros' exponent part is used for looking up a gain value in a
table, and 'frac' is used for interpolating between two nearby table
values. Code snippet below:

  zeros = WebRtcSpl_NormU32((uint32_t)cur_level);
  tmp32 = (cur_level << zeros) & 0x7FFFFFFF;
  frac = (int16_t)(tmp32 >> 19);

In the second line, 'cur_level' is shifted upwards so that the leading
bit is '1', after which the leading bit is cleared. The result is
'frac' in Q31.

The compiler type of 'cur_level << zeros' is 'int32_t'. This is a
fuzzer error 'Left shift cannot be represented in int32_t', 
because the leading sign bit is 1. This CL changes the compiler type to
uint32_t.

Bug: chromium:776286
Change-Id: Ie29552b75e690057bd76fc88e747841b531e3802
Reviewed-on: https://webrtc-review.googlesource.com/14841
Reviewed-by: Alessio Bazzica <eed802d553432598d8dd1392efc4d65b5eefde4d@webrtc.org>
Commit-Queue: Alex Loiko <999d0b481e5703ad7006f0848e249f8b2c3f29bd@webrtc.org>
Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#20405}
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