The unit testing of vulkan shader module crashed since the codeSize
was passed wrongly as length/4 instead of original length in vkCreateShaderModule.

As per specification, codeSize is the size, in bytes, of the code pointed to by pCode.
So it cannot be less than the length returned by std::string::length().

Changed the codeSize from length/4 to length.

BUG = 582558, 582564

CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:linux_optional_gpu_tests_rel;master.tryserver.chromium.mac:mac_optional_gpu_tests_rel;master.tryserver.chromium.win:win_optional_gpu_tests_rel

Review-Url: https://codereview.chromium.org/2388993003
Cr-Commit-Position: refs/heads/master@{#424387}