Don't reuse POST XHRs in the MemoryCache

The HTTP spec states that certain request methods are implicitly uncacheable. Respect that in deciding cache policy.

Also, don't automatically assume that it's safe to merge simulatneous XHRs to the same resource. This is safe in the standard subresource case, but XHRs might have manually set headers that influence caching decisions (e.g., Cache-Control).

BUG=361087
TEST=CachingCorrectnessTest.PostToSameURLTwice

Review URL: https://codereview.chromium.org/231773002

git-svn-id: svn://svn.chromium.org/blink/trunk@171184 bbb929c8-8fbe-4397-9dbb-9b2b20218538
RenderBlock::nextFloatLogicalBottomBelow should allow LayoutUnit::max() as a return value

Update the function to allow return values up to and including LayoutUnit::max().
BUG=356678

Review URL: https://codereview.chromium.org/225023025

git-svn-id: svn://svn.chromium.org/blink/trunk@171185 bbb929c8-8fbe-4397-9dbb-9b2b20218538