Simplify BrowserContext by removing redundant methods.

GetRequestContext duplicates BrowserContext::GetDefaultStoragePartition(browser_context)->GetURLRequestContext().

BUG=159193

Committed: https://crrev.com/f26bfb5f167883133d755704bfab8ab8215959c9
Cr-Commit-Position: refs/heads/master@{#386512}

Review URL: https://codereview.chromium.org/1864583006

Cr-Commit-Position: refs/heads/master@{#386716}
82 files changed