Add CreateReadChunkRequest test to cpp/request_test.cc.
Add CreateReadChunkRequest test to cpp/request_test.cc.

Test CreateReadChunkRequest request. The test makes sure that offsets
greater than int32_t are handled correctly.

BUG=chromium:388077
Change-Id: I9c5aa6683861bb3f5704b54700f4334d3bd9de00

vapier/chrome-ext-wicked-good-unarchiver,vapier/chrome-ext-wicked-good-unarchiver,vapier/chrome-ext-wicked-good-unarchiver,vapier/chrome-ext-wicked-good-unarchiver,vapier/chrome-ext-wicked-good-unarchiver