Add MASM settings support to msvs_settings.

The 'UseSafeExceptionHandlers' setting, if set to 'true', passes the
/safeseh option to ml.exe.

Without it, linking fails with error messages like this:

[182/259] LINK_EMBED(DLL) crnss.dll
FAILED: C:\python_27_amd64\files\python.exe gyp-win-tool link-with-manifests env
ironment.x86 True crnss.dll "C:\python_27_amd64\files\python.exe gyp-win-tool li
nk-wrapper environment.x86 False link.exe /nologo /IMPLIB:crnss.dll.lib /DLL /OU
T:crnss.dll @crnss.dll.rsp" 2 mt.exe rc.exe "obj\third_party\nss\nss.crnss.dll.i
ntermediate.manifest" obj\third_party\nss\nss.crnss.dll.generated.manifest
nss_static.lib(nss_static.intel-aes-x86-masm_asm.obj) : error LNK2026: module un
safe for SAFESEH image.
nss_static.lib(nss_static.intel-gcm-x86-masm_asm.obj) : error LNK2026: module un
safe for SAFESEH image.
crnss.dll : fatal error LNK1281: Unable to generate SAFESEH image.

Add the ImageHasSafeExceptionHandlers setting support for VCLinkerTool. If set
to 'true', it is mapped to the /SAFESEH linker option.

R=scottmg@chromium.org
BUG=none

Review URL: https://codereview.chromium.org/217813007

git-svn-id: http://gyp.googlecode.com/svn/trunk@1894 78cadc50-ecff-11dd-a971-7dbc132099af