win vs2015: fix shadowing warnings in net/url_request

win vs2015: fix shadowing warnings in net/url_request

d:\src\cr2\src\net\url_request\url_request_filter.cc(149): error C2220: warning treated as error - no 'object' file generated
d:\src\cr2\src\net\url_request\url_request_filter.cc(149): warning C4456: declaration of 'it' hides previous local declaration
d:\src\cr2\src\net\url_request\url_request_filter.cc(141): note: see declaration of 'it'

d:\src\cr2\src\net\url_request\url_request_http_job.cc(917): error C2220: warning treated as error - no 'object' file generated
d:\src\cr2\src\net\url_request\url_request_http_job.cc(917): warning C4456: declaration of 'context' hides previous local declaration
d:\src\cr2\src\net\url_request\url_request_http_job.cc(859): note: see declaration of 'context'

d:\src\cr2\src\net\url_request\url_request_job_manager.cc(79): error C2220: warning treated as error - no 'object' file generated
d:\src\cr2\src\net\url_request\url_request_job_manager.cc(79): warning C4456: declaration of 'job' hides previous local declaration
d:\src\cr2\src\net\url_request\url_request_job_manager.cc(71): note: see declaration of 'job'

ninja: build stopped: subcommand failed.
R=eroman@chromium.org
BUG=440500

Review URL: https://codereview.chromium.org/937393002

Cr-Commit-Position: refs/heads/master@{#317384}