Unify WebCachePolicy and mojom::FetchCacheMode

This CL replaces WebCachePolicy by mojom::FetchCacheMode. Having two
similar enums is redundant / confusing, and transferring a mojo enum
is easier than transferring a usual enum over mojo.

 - kUseProtocolCachePolicy => kDefault
 - kValidatingCacheData => kValidateCache
 - kBypassingCache => kBypassCache
 - kReturnCacheDataElseLoad => kForceCache
 - kReturnCacheDataDontLoad => kOnlyIfCached
 - kReturnCacheDataIfValid => kUnspecifiedOnlyIfCachedStrictly
 - kBypassCacheLoadOnlyFromCache => kUnspecifiedForceCacheMiss


Bug: 453190, 624278
Change-Id: I9b08cc47af1769960053cc540a7db5c6a1c3820b
Reviewed-on: https://chromium-review.googlesource.com/724563
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Jochen Eisinger <jochen@chromium.org>
Reviewed-by: Takashi Toyoshima <toyoshim@chromium.org>
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Reviewed-by: Kouhei Ueno <kouhei@chromium.org>
Commit-Queue: Yutaka Hirano <yhirano@chromium.org>
Cr-Commit-Position: refs/heads/master@{#511149}
52 files changed